A 20 MOa does not always give you 20 MOA, but close to it. So if you have 28.5 MOA from bottom of scope to your 100 yard zero the rail will remove between 16-20 MOA of that. Meaning your zero will be between 8.5-12 MOA on your scope leaving you with 41 - 45 MOA on your scope. As you only need a max of 36 MOA with ANY 30" barreled rifle any most any target ammo you will easily make the distances with a 20 MOA rail.

ALso a 30 MOA rail not allow you to come down low enought o attai a 100 yard zero. Can be important for returning to zero for comps, rifle checks, etc all of which are done at 100 yards usually.

As to HPS. The average MV from Carmichael's is usually around 2950. Having chrono'd some rounds they averaged 2917fps with some dipping below, and other above. Heat, cold, headwind, etc all play a part so the 33.5 MOA stated above, and used as the basis for the 25 yd safety card is applicable to the day it was shot on.

The top of some actions are flat. So when a 20MOA rail is mounted with the rear of the rail higher than the front its obvious.

Other actions though are slightly "sloped" on the rear of the action so require a slightly higher rear height than front height to keep the rail (where scope mounts) level. So even though the rail looks inclined it is actual not a 20 MOA rail but 0 MOA.

All that aside. If you are using 28.4 MOA to achieve a 100 yard zero i guarantee the rail is not sloped/inclined. It is a 0 MOA rail. No .308 target rifle fitted with a 10/20/30 MOA rail takes that much adjustment of the scope to find its 100 yard zero.