It would be an easy problem to solve for how much weight the smith is taking off, you'd just need the angle that its at. You would have an x and y equation is all. I'm not exactly sure what you guys mean by taking the weight off though Smith might be a crutch but sometimes a crutch is a good idea when safety is involved. If it was a matter of you benching heavy on a flat bench without a spotter vs. heavy on the smith I'd much rather see you on the smith. Or you could do dumbbells
If its an ~80 degree angle and you have 800lbs on then that would be 470lbs in the y and 647lbs in the x. y being vertical and x being horizontal. So you'd be doing somewhere around 650lbs. Been a while since I did dynamics and I did that in my head so it might be wrong if someone wants to actually know I can model it...