Are you doing the BIAB kit? The BIAB is a 3-gallon kit. Or are you doing the 5-gallon kit using a BIAB process?
The instructions for the 3-gallon kit say to start with 5.5 gal, mash in all of it, then pull the bag after the mash and drain the bag into a bucket.
Then add liquid from the bucket back to the kettle until you have 4-gallons.
They presume you will have 1.5 gallons absorbed by the grain, and an additional gallon lost to steam during the boil.
I don't follow their instructions. I mash with 3-gallons, drain, then "sparge" with almost 2-gallons. My stove boils off about 3-qt/hr so I look for 3.75 gallons pre-boil.
One thing I might recommend before attempting either kit is to fill your kettle with 4-gallons of just water and boil it for an hour. After the hour, see how much is left. That will let you determine your boil-off without out wasting ingredients. If your pot doesn't have volume markings, put the lid on until it's cool enough to measure.
However you get there, you need to start the boil with the kit's target volume, plus the amount you boil-off in an hour.