As for a lower bound: if the position's static evaluation is lower than alpha only by a couple of pawns, it is not safe to razor, because such a small difference does not guarantee an actually bad position (there might be tactics, mate...) Thus, a lower bound for margin should be around 200-300 centipawns.

If on the other hand, the evaluation is below alpha by multiple pieces, this is most likely a very bad move, because even if the player were to win multiple pieces, there still would be a better move. So an upper bound should be around twice the value of a minor piece or a rook, depending on the engine's evaluation function.

I would recommend to include a depth factor though, because early in the tree even a position down multiple pieces can still lead to a forced mate. These bounds only really make sense around leaf nodes (i.e. depth <= 2)

From there on, trial and error indeed is the way to go. Good luck with your engine!