When doing a full boil what happens when you start with too much water and don’t boil enough off? Say I started with 7 gallons and boiled for 45 minutes and had 6 gallons at the end of the boil.
Add DME to raise the OG. What was your target if it was 5.5 unless it's super low maybe just leave it. What are you making?
First depends on what the recipe calls for.
Second, if it’s 5gals then you have a low OG. Your bitterness is also going to whacky.
As @brew_cat said you could add DME to bring up the stated recipe SG.
Or you could boil it longer/harder. As stated above the answer really depends upon the beer and how you want it to finish.