You are talking about programming the CV for starting voltage? I can't speak for the actual decoder that you have, but on the digitrax decoders I use, the start voltage CV can be set between 0 and 255 where 0 equals 0 volts and 255 equals maximum volts. To figure the actual voltage, you would use some ratio mathematics. On the other hand, knowing the actual voltage isn't all that important because the CVs that control the maximum speed, the midrange speed, and the start voltage are all programmed from 0 to 255. It is a trial and error process to change these variables to make the loco run like you want it to.
For example, on my Athearn CF-7 with a digitrax decoder, typical values would be 35 for start voltage, 110 for maximum speed, and 70 for midrange.