...Also I do understand why you want to measure from the middle of the keg?? Where is the beer being sucked from?? Not the middle. It is being sucked from the bottom. Micro Matic is the only maker of these stems and valves, and you want to measure from the bottom of the keg to the faucet.
They take from the middle of the keg because the liquid will add some hydrostatic pressure to the keg since it is drawn from the bottom. The reason to take the middle is because for half of the keg the assumption will be a bit low and the for the other half a bit high, but all in all it will work out not to bad.