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When you're developing a new recipe. Can you predict how much water is displaced by grain. And can the absorption of water in the grain slightly make up for the loss. I know you will loose some volume to evaporation, though minimal. I haven't really found a great answer. I have found that grain will absorb some water and a basic rule of thumb. However, it is generalized and not grain specific.

Normally I wouldn't care but I want to improve mash efficiency and get a better conversion.

Try looking at brewing sites. But then grains differ. For corn meal my numbers are. 20kg meal in 80lt water (on grain fermentation) will net me 68lt of beer after I have pressed for 8h. The recovered meal then weighs around 15kg. Most of that weight is the beer that i cannot recover with my press.

One of the brewing software packages I use to calculate this uses (0.12 gallons) / pound of grain. This is primarily barley and I don't squeeze the grain. Some others say as high as 0.2 gallons per pound. I suspect that this is the rule of thumb you already know. Since the OG is important for beer making I check the SG of what I collect and adjust via water additions and boil off. In my experience sugar extraction efficiency has been more related to grind of the grist and not water ratios.

Sometimes on an AG mash I'll siphon down to the grain-bed for first strips then top up with water and repeat for a lower ABV second strip (still stripping to average 30%abv or so) - Then do my sugarhead or second ferment before squeezing... I try to get two ferments out of an AG mash just to stretch it a little - reserving that first ferment low wines for the top shelf AG. In my mind the amount of grain (and related mash efficiency) determines the amount of low wines since the water ends up being left behind in the backset...

Cheers!
-j

Experience will help with this. Beersmith is a great tool for all grain mashing, it will give you the info on this plus all your numbers for the recipe. Should be free trials.

I have found on all grain mashes to plan on losing 3 gallons of water out of 6 so, i started making 7 gallon mashes using an extra gallon while sparging for a total of 8 gallons of water to ferment 5 gallons off grain