We have a mash tun which holds 20gl beneath the false bottom. Do we incorporate this amount of liquid as mash liquid with respect to mashing ratios or will it dilute the resulting wort if we don't?
If a recirculating mash is used this is added to the volume in water grist ratio.
If the volume not in contact with grain remains largely unmixed it can be purged after the mash and omitted from the water grist ratio. But no matter how undisturbed this water is you will lose some fermentables in the purge reducing mash effeciency some.
20 gallons Is a lot. What is your total volume of the mash tun? Usually this kind of dead space is only in commercial systems.
In either case if the false bottom volume exceeds about 20% of the total volume you will start to seriously effect efficiency.
I would include it, as the lower layers of your mash will diffuse into and out of this volume. I am guessing if your dead space is 20gal ~ 1HL, then you are brewing with some seriously large home brew kit or some tidy professional equipment.
There are other reasons you should include this volume in your calcs, you will be stirring/using a grist hydrator so this volume wil dilute the diastatic strength of the solution in your mash tun; as mentioned especially in the lower portion of you mash.
This will end up in your final wort and in my experience I get a lot of fine milled grain sitting in dead space which converts and is converted making this dead space rather rich in extract. We occasionally have to apply back pressure to re-float the mash, or sometimes recirculate to filter out this floury mess.