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Is tire pressure to weight a linear equation? Pressure often moves as a square; see Boyle's Law.
Well, it's P varies with the inverse of V. So unless one axis uses x^-1 you'll get a hyperbola. Either way, the graph you made is likely a decent approximation considering the small variations and multiple variables we're dealing with. Though, according to Dave Moss, we're both wrong: https://www.youtube.com/watch?v=XaIuWKfnGEIreference for hyperbola: https://en.wikipedia.org/wiki/Boyle's_law
DrM, you shortchanged yourself by 84 lbs on the rear LI.
GAWR is 686 lbs rear and 322 lbs front at 41/36 psi. You state the front is maxed at 399 lbs (77 lbs too high) and rear maxed at 599 lbs (87 lbs too low). Max total weight (GVWR) is biased more to the rear at 68/32, not 60/40%.
The max loading specs are inside the right down tube. 68/32% is what they work out to. If you use saddlebags and a luggage rack, the combined load will be slightly behind the rear axle. The sissy bar allows the passenger to lean back so their CG is roughly right over the rear wheel. I used your measurements of weight distribution to plot the minimum tire pressures and Yamaha's specs for the maximum. A light rider with no passenger and a heavy windshield but no other accessories or added weight (58/42%) might need 36/36 psi. A light rider with a heavy passenger/loaded saddlebags and loaded luggage rack (70/30%) might need 41/33 psi. Hopefully, you'll be able to get a pretty good idea what combo is best just looking at the distribution and estimating about how heavy things are.