I made an updated spreadsheet that I think does a better job of computing tensions in the chains. This is a statics problem and the equations are a little unique.
how are your euasions different from the existing spreadsheet (and why are they
The force calculations use statics equations instead of the vector calcs. In all cases, the left and right forces on the sled should equal, otherwise it would be moving. I illustrated those calcs in the graphics. I left out the derivation of the main chain tension calc, but can provide it if needed.
so a standard setup 10’, 18" rise and 20 lbs sled has a high value of 34.8 and a low value of 3.27 with this new spreadsheet and the old spreadsheet has 34.80 and 3.84.
with 12’ beam 30" rise and 25 sled new spread sheet says 33.8 and 6.67 while old is 33.8 and 7.95
so basically your new spreadsheet says the forces in lower corners is 17-19% worse then what we thought before.
Yes, that is correct. The lower left and lower right corners are the most unstable X locations.