I have inserted a reference plane as shown in the picture below. I'm trying to specify the distance to a few decimal places. At first it would only accept integer numbers. Can anyone tell me why? It somehow fixed itself but I want to understand what happened.

But what I really want is to enter a formula or variable because the distance is related to the length of the base of the pyramid. Is there a way to do that? I think I could go back to my starting 3D sketch (dashed lines in the picture) and set a reference line if this doesn't work.

Thanks everyone.