I'm on my first batch of wort, I didn't account for evaporation and had a couple boil overs, I'm sitting at about 4.25 gallons of wort. Bottling calls for 5oz of priming sugar, should I reduce to account for the lost .75 gal.?
Yes, you should reduce the amount linearly. 5oz * (4.25gl/5gl) = 4.25oz.
EDIT: on second thought, 2.5 volumes isn't all that high.
Though: 5oz of sugar will result in a pretty high carbonation for 5gl of beer. What sort of style? At what temp will you be doing your bottle priming?
I suggest using a priming sugar calculator to get a better handle on how much of what type of priming sugar to use for your batch.