This will be much more intuitive if you move on to instrument training, where referring to the climb/descent rate gradient will seem like a daily thing.
POHs are different. A 172N for instance, gives only a speed and a graph. Others will give speeds calibrated by weight. Some will give glide ratios. But often in parametric physics equations, you have a few known values that you have to then convert into additional "intermediate" known values, that you only then can convert into the answer you need.
So, 172 as an example... Best glide is 65, and I can tell by the graph that I'll cover about 1.5 NM per 1,000 feet of altitude. Let's get to work!
1.5 NM = 9,000 feet, which means my glide ratio is 9:1, or expressed as slope is 1/9 (rise over run). We don't need to know the degrees of slope, but if we're interested it works out using the tangent formula to be 6.3 degrees.
If I am moving at 65Kts (remember this is indicated, and both calibrated and GS will be different) then lets treat that 65 as horizontal motion. Yes, technically it is the hypotenuse of a 1:9 triangle but the math required isn't justified by the fraction of a knot it will correct. So at 65 Kts, I am covering about 6,500 FPM horizontally before compensating for wind. Beyond a few thousand feet, miles are more useful than feet, so 6,500 feet is 1.083 NM.
I know from my glide ratio that if I am covering 6,500 FPM, a 1:9 ratio means I am descending at 720 FPM. I can solve this just dividing 6,500 by 9, or I could have used the Pythagorean formula in step 2. But at these glide ratios, division is accurate enough.
How long I will be aloft will therefore be AGL / 720, in minutes. Or that answer times 60 if you need seconds.
How far I glide will be the time aloft (answer 4) x 6,500 feet, or if more useful in NM, it's time aloft x 1.08NM.
Now I have all the info I need to actually create a table, remmebering these estimates are in NO-WIND conditions.
|Glide Dist NM
|Glide Dist Ft
This is KIAS which at slow speeds will be slower then your KCAS, see your aircraft's calibration table in Section 5. At 65Kts a 172 only has about 1-2Kts of error at most, but a 182RG could be 4 knots, and that's material to a glide plan.
Wind can make your groundspeed VERY different than KCAS, and in any ground-reference maneuver, you MUST take wind (and thus GS) into account. And a gliding power-off landing is definitely a ground-reference maneuver, LOL.
This does not take into account the drag and load factors of a turn. These are HUGE considerations in glide descent planning. The POH assumption is always "land straight ahead, and you hit what you hit."
This does not take into account recognition time. You can eat a lot of speed or a lot of altitude in just the few seconds it takes you to interpret the problem and commit to action.
This does not take into account your amount of rehearsal. Recency of practice of power-off returns to field has been determined to be the single largest determinant of success, even for experienced pilots.
Finally, remember that any extra speed, altitude, or wind toward the field that is occurring at the key position increases your odds of inadvertently over-running the runway and going off the far end. We get so focused on protecting the glide that we sometimes forget we still have to land.
Hope this all helps!
Adding some PA38 Specific information to better suit the OP
Here's a "typical" glide performance table from the PA38 POH. Piper publishes this detailed table in section 5 (Performance) of your POH, whereas Cessna provides only a cursory graph in section 3 (Emergencies).
Some of the comments on this thread imply it's somehow a legal violation for you to have access to glide performance information, and I find those comments to be unacceptable and have stated that clearly. The information is right in the POH, there's no secrets being revealed here.
You have to be careful about using this information only to form your own "intuition" about how your glide performance changes over a range of conditions, and not as a specific glide ratio you calculate at the moment of engine failure, as there won't be time.
What this chart shows is that at standard temperatures your glide ratio will be on the order of 7.5 to one, which is somewhat steep--which is what we'd expect to see with a glide speed of 70 Kts.
At 70 knots you are moving forward at roughly 7,000 FPM horizontally (no wind), and a 7.5 to 1 glide ratio means that you'll be descending in the neighborhood of 933 FPM. That means that from 2,000 feet you'll be looking at just over 2 minutes aloft. This is useful information to help think about what remedial actions I might have time to take, such as a restart attempt, but can't and must not be used for real "will I make the clearing" calculations.
This chart also tells me on a very hot day, say 10 deg. C above standard, I might see glide ratios more like 10.5 to 1, which is about 666 FPM vertically. From 2,000 feet AGL I now have 3 minutes aloft and cover about 3.5 NM before wind.
This also means that on very cold days, your descent will be steeper at 70 KIAS, and your time aloft will be shorter and distance traveled will be as well.
I absolutely encourage you to spend some time with your instructor both at the white board and in the airplane to determine under what conditions you might adjust your glide speed based on weight and atmospheric conditions. But that's not an exercise for strangers on the web.
Remember, there are many factors
The factors include weight, density altitude, load factors if turns are required, recency of practice, atmospheric conditions, And WIND, WIND, WIND, WIND. The purpose of this exercise is to give you a mental picture, an intuition that serves as the starting point for your own thinking about how various atmospheric conditions affect glide performance.
As you move to other aircraft, the dynamics will change a lot and it's always worth spending some time with your instructor really digging into the performance tables and understanding "so what does this all mean."
Always, always strive to understand the factors that uniquely affect the performance of every single aircraft you fly, in every atmospheric condition that is common to your region.