Sure a partial boil works. Many brewers do it for BIAB as well as extract. I prefer a full boil and once you've done it a few times and settled on your methodology you'll know your boil off rate and be able to more accurately determine what you need.
I do a kind of modified 2 vessel BIAB with a sparge. Began using the process when I needed to mash thicker due to a small kettle but kept it up since I get such great results.
I usually brew 10 gal batches but whe I brew 5s I target 5.25 gallons into the fermenter so I target about 6.25 preboil volume. I mash in my BK/MLT with 4.5 gals which generally get's me around 1.25 qts per pound of grain. I heat another 6 gallons or so of acidified water in my HLT for sparge during the mash. When it's done i pull the bag and suspend it over the BK, turn the burner on, run a silicone hose in the top of the bag and pump sparge water through the grain bag until I get my boil volume of about 6.25 gals.
Your boil off will depend on how hard you boil as well as some environmental conditions like altitude and humidity in your location. While you're figuring out the correct volume for your system and process I'd shoot for less since you can always top off in the fermenter. if you reach the end of your boil and your volume is high you could continue to boil so you have a better chance of hitting your target OG but that will affect your hop utilization and bittering as well.
It's all really trial and error to dial in your specific process. If you've been doing extract your should have some idea what your boil off rate is. If you did full volume boils for extract shoot for the same preboil volume.
Just try to be consistent with how hard you boil.
EDIT: The 6 gallons of water I heat for sparge is overkill but I will boil the 2-3 gallons left over then pump it through the system and use it for clean up after the brew day is done.