Lauter efficiency
IS (almost) completely independent from conversion efficiency. Mash efficiency is completely dependent on both lauter and conversion efficiency, and nothing else.
My spreadsheet does not attempt to predict FG. There are too many interacting variables, that are different for each brewer's system, to make any theoretically sound predictions of FG. All FG predictions are based on a bunch of curve fit correlations that are usually acceptably close, but can frequently be wildly off. My spreadsheet (and others' similar calculators) are based on simple dilution theory, and doing mass balance calculations. There are some fairly simple to meet constraints (assumptions) that must hold for the results to be really accurate, but the calculations can provide quite a few useful insights even in cases where the constraints are not met. The only curve fit correlation used is the equations that translate between Plato and specific gravity, and those are pretty damn accurate.
The only thing that isn't really rock solid is the assumption that the grain absorption rate is the same for every run-off. However, if you have data that shows different grain absorption rates for different run-off steps, those could be incorporated into the calculations. I haven't bothered since the other measurements have enough measurement error that the efficiencies calculated only have an accuracy of +/- 3 - 4 percentage points. Any efficiency calculator based on homebrewer volume and SG measurements, and relying on historical, rather than actual lot, data for grain potential and moisture content will have the same level of uncertainty. Some are known to have larger errors due to incorrect application of formulas or definitions.
I think I've gotten far enough off track, so I stop now.
Brew on