Pots value on a strat with Humbucker on bridge

Stock single coil Fender's are typically 250k...increasing the value typically makes them too bright (subjective).

Humbucker's are typically 500k...decreasing the value typically makes them too dark (subjective).

Maybe try splitting the difference with 300k and hope for the best.

 

I've always used the stock 250k's when I've dropped in bridge humbuckers.
Always sounds fine to me.