I do BIAB with a 55 L (15 gallon) kettle so I CAN to do a full volume mash. That makes a difference in the volumes but it may be as simple as you say. Calculate for less mash volume then just top up to your pre-boil volume. Keep in mind your gravity at the end of a partial volume mash should be ABOVE your target pre-boil volume because you dilute it when you top up.

The amount of grain you use will also effect your volumes due to absorption. I use about 5-7 kg (11-15 lbs) every brew so I can keep things at a pretty close constant. (It took me 3 brew sessions to make sure I had all the volumes to where I liked the outcomes.)

In the end it just takes repetition and some good notes and you will nail it for your equipment and process...until you upgrade equipment

 

I'm another full volume masher. I just prefer the simplicity and I don't notice much of an efficiency hit.

Though I think there's probably a point where the mash may be too thin. For my low ABV beers (I do a few around 3% ABV), I'll top off with water. If I was low I'd leave the bag in the kettle, but haven't needed to do that often. Currently I try not to go thinner than 5, maybe 6, litres per kilo of grains.