I am always confused about how to calculate safety altitude, especially when it comes to even heights and odd heights, e.g. is
7500ft even or odd?
See below for the wording of 14CFR91.159 which is the reg you need to answer this question. Note it says "Odd [or Even] thousand feet plus 500". 7000 is an odd thousand even though it is an even number.
§ 91.159 - VFR cruising altitude or flight level.
Except while holding in a holding pattern of 2 minutes or less, or while turning, each person operating an aircraft under VFR in level cruising flight more than 3,000 feet above the surface shall maintain the appropriate altitude or flight level prescribed below, unless otherwise authorized by >ATC:
(a) When operating below 18,000 feet MSL and—
(1) On a magnetic course of zero degrees through 179 degrees, any odd thousand foot MSL altitude 500 feet (such as 3,500, 5,500, or 7,500); or
(2) On a magnetic course of 180 degrees through 359 degrees, any even thousand foot MSL altitude 500 feet (such as 4,500, 6,500, or 8,500).
In the US there is a simple rhyme memory trick that you can use to figure out what altitudes would be valid for you to fly both VFR and IFR.
EAST IS LEAST - WEST IS BEST
What that means is that you fly odd number altitudes when flying East (Magnetic) and even number altitudes when flying in a magnetic Westbound direction. For VFR just add 500 ft.
So valid West bound flights would be: 4000,6000,8000 IFR : 4500,6500,6500 VFR
Same with East Bound: 5000,7000,9000 IFR : 5500,7500,9500 VFR
A simple rhyme that works pretty well for me. Also BTW, if you are filing then chose the initial leg as your filing altitude. If you are under ATC control, you'll see that ATC will eventually ask you to climb or descend if your course leg changes your direction. IF VFR, and you make a bend that takes you to the other side, then climb or descend even if you are not talking to ATC. If talking to ATC, let them know. They can either tell you to stay put or pilots discretion to the new altitude.
The reason for this is that head on's are pretty hard to do if everyone is flying the same direction. You might get chewed up in the tail, but that's the job of the pilot in the plane overtaking you or ATC.
I don't find any of the other answers to provide a useful mnemonic, which is what you need for rote memorization of arbitrary rules like this. The one I learned (and still remember decades later) is this:
ONE = Odd for North and East
In other words, if you're on a North or East course (360 - 179), then use Odd thousands.
By elimination, if you're on a South or West course (180 - 359), then use Even thousands.