Am I correct in assuming that you are using the Glideslope Indicator to measure your angle of descent. In other words, the wording of your question makes it sound like you believe the center of your Glideslope Indicator represents an angle of descent of 0°, and the next dot represents a descent angle of 2.5°.
The angle of descent of an ILS is set by the position of the transmitter equipment on the surface of the airfield. It is something that remains constant, and does not change unless someone on the ground adjusts the actual antennae on the surface of the airfield. The normal position of the ILS antennae are that the Localizer antennae are located at the departure end of the runway, facing the arriving aircraft. The Glideslope antennae are located along side the runway, normally abeam the precision approach aiming point (thousand footers), angled upwards approximately 3°. You can not change this from the aircraft.
Your goal is to keep the Glideslope Indicator centered during your entire descent from the Final Approach Fix. Some pilots will choose to intercept the Glideslope before descending from the Intermediate Fix altitude. In which case, you keep the Glideslope Indicator centered from there. If your indicator is positioned on the dot above center, you are flying below the 3° glideslope. If your indicator is positioned below the center, you are flying below the 3° glideslope.
Always intercept the Localizer before intercepting the glideslope. Beware trying to intercept the glideslope by descending into it from above. Nor should you try to intercept the glideslope before the Intermediate Fix or at an altitude above the Intermediate Fix altitude. You may encounter false signals, otherwise.
One trick to this is to have your aircraft fully configured prior to reaching the Final Approach Fix. This includes:
- Gear lowered. Prop governor at the highest RPMs. And, flaps set for your descent.
- Aircraft trimmed for your desired approach speed (preferably the ground speed indicated on your approach chart to time the Missed Approach Point in your aircraft speed category).
- Power reduced and set to maintain a constant descent vertical speed for a 3° glideslope. This speed may be listed on your chart (depending on whether you use Jepp, NACO, etc.). A good rule of thumb is to multiply your ground speed in knots by 5. This will give you your vertical speed for a 3° descent in feet.
- Switch on landing, position, and strobe lights. Adjust mixture for maximum power setting. Turn on the boost pump if required by your POH. Fuel selector on the proper tank. And, open carb heat and/or close cowl flaps if so equipped.
- Stabilize pitch, roll, lateral direction (compensating for wind), power and airspeed (adjusting for half of gust factor).
- Start doing all of this 1-2 miles before the Final Approach Fix or when the Glideslope Indicator is a full to a half dot above center. This will give you time to complete everything in time to intercept the 3° glideslope stabilized.
- Make your Final Approach Fix radio call to ATC or area traffic.
- Maintain the lateral and vertical guidance ILS center using small, smooth movements of the control yoke. If set properly the first time, you should not have to adjust the throttle until you are preparing to round out for landing or go-around.
- Keep the ILS centered to within half a dot at all times. If you reach a full dot of vertical guidance, readjust your throttle. If you are at or beyond a full dot at MDA(H) or DA(H), either go-around immediately at the DA(H), or prepare for a go-around at your MAP.