Precision of incline angle?

We have many bed sizes here, and after getting an inclinometer find some interesting issues. With a 6 inch...I.e. 5 inch indent...I got 3 different angles on 3 different beds...3.5 deg, 4.5 deg, and 4.6 degrees. If I remember correctly passing 5 inches starts giving the full benefit...6 inches is to make sure. A UK bed means that's around 4.5 degrees. 5 degrees to make sure. Looks like I need to get the larger bed risers then. Any comments on this are welcome. Of course there are benefits from even 3 inches, especially with traction....but one Ned's to feel the real effect.

Yes, your right, beds come in all sizes so bed raisers need to be correct height for maximum benefit. Place timber under bed raisers to correct it?