I for a full bevel set I typically start with 30 and work my way down through the grits. The trouble with films is that you can dull the sheet if you spend too long on them. I typically do not reuse lapping film sheets. You don’t get this problem with stones. Starting at 12 may be sensible but you might need a couple of sheets depending on how much work there is to do.
Here’s a shot of the back of the Shapton Glass plates. That will give you a rough conversion from micron to grit rating.
Might be worth the investment getting a stone for bevel setting depending on how often you do it. 12 micron would probably be the lowest I go unless there are chips.