Setting up Brewtarget for partial boils

So far, I’ve done 2 all grain BIAB batches on my stove top in a 5 gallon kettle. I like the results but I really want to get more out of the time I spend brewing. At the moment, I mash with 3 gallons of water and by the time it’s all said and done, I have about 2 gallons of wort. this give me a little less than a case per batch. It might not make enough difference in the calculations, but how does one set up Brewtarget for partial boils? I only intend on topping up with 1 gallon before fermenting.

I MIGHT be able to add another gallon to the mash but I’ve only been doing session beers so far. Anything bigger and forget about adding more.

Sounds like you’re kinda doing what I’m doing, but I’m using a second pot to do a sparge. Meaning I mash in one 5 gallon pot, usually about 3.5-4gallons of water. I’ll fill a second 5 gallon pot with about 2-3 gallons of sparge water. When my mash is done I squeeze as much wort from the grain sack as I can. Then I dunk the sack in my sparge pot which I have at around 170-175 (so the temp drops to the mid 160’s after the grains are added). After 10min I left the sack from the sparge pot and squeeze as much wort as I can out again. I then bring both pots to a boil separately (just because it’s quicker than all together). I pour the 2 into one pot and top off with fresh water until I’m below the rim which is about 4.5-5gallons. After an hour boil I’m down to 3.5 - 4 gallons of wort where I then just top off with fresh cold filtered water to get me to about 5 gallons.

I’m about to buy an outside burner so I can do full boils and get the wort boiling quicker (takes forever on an electric stove top), but my current setup allows me to do mashes of up to about 13lbs. So I can do beers up to about 1.055-1.060 OG. Anything higher than that and I either do a smaller batch, maybe 2.5-3 gallons or I add a little extract during the last 15min of the boil.