Does your mash water come directly from your HLT? If so then you would add 3.75 more quarts to compensate for the losses (i'm assuming you're also compensating for grain absorption). Then for the sparge you wouldn't need to compensate anything because all that dead space has already been filled.
Determining how much water to use is not really an exact science, more estimating. Especially if you're fly sparging. Like I will mash at 1.4 quarts/pound, collect runnings, then fly sparge until i collect enough wort to hit my target volume post-boil. If i'm batch sparging, I estimate that I'll get back 95% of the water I put in. For me, about 7.7-8 gallons is sufficient for a 90 minute boil. It's also better to overshoot than undershoot. You really don't want to add water at the end of the boil.