I brew on a system that is similar. After adding the water under the false bottom, I then calculate the amount of water needed for mashing above it. The total amount of water is used to calculate the strike temperature, so if you figure 1.5 quarts of water per pound of grain, but have an additional 4 quarts under the false bottom this increases the percentage of your calculation for accurate mash in temperature.

Using a fly (continuous) sparge and assuming the dip tube (pick-up) is at the deepest part of the keggle, the liquor drawn off to the kettle should be fairly efficient.