I realize things are seasonal, and I also realize I live in a climate that is usually quite humid. Having said that, and with my super simple BIAB. I start with 8 gallons of water. I know from experience that the bag will eat about a gallon, and I know from experience that my boil off is about a gallon an hour. That leaves me with 6 gallons and 5-5.5 in the fermenter.
If you don't have the lines on the kettle to measure, use an unscientific method: put measured water in it and make a few marks. If you have brewed a few batches, you should have an idea of boil off and what else reduces the water level. From there, adjust the software or boil longer or add water at the end, or all of the above.