Spotfin Questions
Q1 - "How far in, inches or mm, do the eggs of the SFS "usually" go into the crevasses, from the edge of the two platters making a crevasse inward towards center?"
I mostly use CD disks and I see eggs inward to the central post which is 2".

Q2 - "Q) Did the distance from the crevice edge to the internal stop, the PVC or threaded rod or whatever center support edge. Did that distance make any difference in the selection of crevice spawn choice?"
Distance of outer edge to internal (stop) post does not seem to make a lot of difference for placement of eggs in my experience. However I have not done a lot of experimenting with this topic. Range of diameter or width of disks or plates that I have used varied between 3" to 6".
Egg placement IMO is primarily based on width of space between disks or width of the crevice.