Hi, the boiloff rate and absorbtion are both going to depend on your equipment / process.
You can easily get your boiloff rate just by filling your boilpot and boiling it for an hour. Water at start - water left after an hour is your boil off amount per hour.
For absorption, I did some research on thw net, and I use 1 liter per kg. I squeeze the bag. There are a couple of good BIAB forums around, just pick a figure and try it.
Keep good records and adjust these (and other) figures as you brew more.
The biggest problem I had with my calcs when I first started was not allowing for trub losses which are quite high in BIAB.
There are a lot of water calculators around, when I first started I used the one on here. Under your profile there is an 'equipment profile' tab. Set that up and go from there.
Good luck!