FRL Calculation question

Hey all, I’m trying to model implied probability for post-cut low rounds. something like (first round leader %) * (% make the cut) * (bonus multiplier for smaller field).

Wondering if anyone has a good method for approximating what I’m calling the bonus multiplier. Right now, I’m using a constant*FRL% where the constant gets the sum of the total field to 100%.

Or, if there is an alternative methodology that I’m just missing entirely.