For the time it would take you to boil water, you could better use that time and actually make beer!
I would aim on the low side of your pre-boil volume, calculate what you boiled off in 60 minutes, and if you are low just add back some water. This strategy might impact your beer if doing an IPA where hop utlization would be an issue, but most other beers you should be OK
Alternatively, if you have too much volume, you can still calculate the boil off at 60 minutes and just continue the boil until you are at volume. In this situation, you would want to adjust your timing on the aroma/flavor hop additions.
Either way, I wouldn't bother with a "pilot" run with water. Just make some beer instead and adjust on the fly.