how far in should i put the rails on a 8.75" board?
The average is about a finger width in from the edge, but it all depends on what you want and the rails themselves.
If you want only rail contact, usually place them on the board and check to see if the middle sticks up (board graphic up) as it will depend on the rails, as some are taller, some more low profile.
If you want super tech slide up into smith / feeble / hurricane or any combos, usually in a bit more, up to two fingers in from the edge - some people even ran them three fingers in from the edge back in the day, but that was on wider boards and it felt really extreme to stand on it like that.
If you would rather they are there to balance you out fully, then people put them almost on the edge of the board, but that is often more for grab tricks, which I don't think you would be needing.
I think that is about it for rail placement, but others might have different takes too.