If you need to increase the gain greater than 1024 (1024/256 = 4) without losing resolution then simply add 1024 to the ADC pot value (range 1024-2048).
If you still need more gain then add 2048 to the ADC pot value (range 2048 -3072).

I do agree with Henrik though, don't worry too much about dropping a digit after the decimal point.
Perhaps use the multiplication method to quickly get you into the ball park and then use the addition method to fine tune the control if required.
This is the method I used to tune my motor speed controller PID loop.

Cheers
Barry
VK2XBP