deadspace and boil off or evaporation rate are the 2 unknowns with any new boil pot, and if brewing outside ambient temps and wind can effect the evaporation.
All you can do really is guess reasonable values to estimate the correct volume to use,
iirc a kilo of grain will absorb 2.2l of liquor, So when you remove the grain bag after the mash it will take circa 2.2l of liquor with it, weighing the damp grain mass with luggage suspension scales will help confirm the loss of liquor due to grain absorption

1l of water is just about 1kg in mass.
so if you using a 4kg grain bill expect to loose 8-9l of liquor when you remove the grain.
Yes the grain bag will triple+ in weight on the way out
Then you have the deadspace if any in the boil kettle, which can be estimated by a simple water drain and measuring any remaining volume not drained.
Then there is the loss due to hop absorbtion and trub, which at about 200ml per 100g of hops is only really valid if the calculated volume is greater than The deadspace Or if you bag and remove the hops..
So Target volume + boil off volume + volume absorbed by grain + deadspace (+ hop/trub volume if relevant) So
23 +4 +9 +Deapspace = 36l+deadspace to start with..
But the actual result will indicate any error in the estimated boil off, if you boil off more you can liquor back with cold water to hit the target volume,
And if you underestimate the boil of rate you can boil just a little longer it will impact on the final bitterness as your hop utilisation will probably increase a tad but i wouldnt let that wory you,
After a couple of brews you will know the nuances of your kit and will be able to predict the boil of rate with a high degree of confidence
But imho volume targets are secondary to gravity targets, and if the gravity post boil is within the target range I dont bother adjusting the volume
