try PM-ing Spillsmostofit,
He uses Beersmith and is a somewhat prolific BIAB brewer. He might have done something. I cant really help with the computer angle I'm afraid, I use promash and just do the calculations manually. Here they are if you want them.
water into kettle = (desired post boil volume) + (boil off rate x length of boil) + (weight of grain x 0.75L)
The total volume of your mash will be the amount of water plus 2/3rds of a litre for every kg of grain. So you just need to adjust your batch size if your total volume overfills your vessel. You can however fill it right to the brim during the mash, when you pull out the bag, it will leave plenty of room to boil without any real boilover problems.
Hope that helps a little, if not, try Spills for possible pre-configured Beersmith stuff, which is of course what you actually asked for.
Cheers
Thirsty