If you take a sample every 10' over 30' - 90', you will see good dmin and then fog in the series. The one before the fog starts is the level you want. If you take samples every 5' or more often, your accuracy will improve.
We used to use small porcelain sample dishes or ultra pure blotters for this test. A drop of emulsion in the dish or dipping the blotter and then developing would give us the fog level in the dark.
As for sulfur to silver ratio it is milligrams / mole of silver itself, and you assume that 100% of the silver nitrate is made into silver halide and is therefore present in the emulsion. The number of moles of silver therefore remains constant throughout, but the volume of the emulsion and the % gelatin will change due to the washing step if you use one.