Oceana - That's what I do, but it takes a lot of trial and error. I usually have 20-30 separate bmp files I have to adjust.
I'll enter 1.00, then I'll select the up-arrow then the down-arrow to view the result (result is not instantaneous).
If the result is not good enough, I'll enter 0.75 (up and down arrows again).
Then 0.60 (up and down arrows again).
Then 0.65 (up and down again).
Maybe I'll enter 0.60 again, or I might try 0.63 (up and down again).
I was hoping there was a trick like highlighting one of the digit places and selecting the up or down arrow to increase or decrease the highlighted digit. Or maybe there was a hidden setting in the preferences somewhere.