I suppose it would be possible to get some idea of the forces from the spindle horsepower needed for a particular set of cutting conditions, but I get an estimate by a bit of a hokey way. The maximum servo force on the X and Y axes of most of my machines is around 2000 lbs, I rarely see the servos operating at more than 10% to 20% load on a straight milling cut on aluminum, steel might go up to 40% sometimes. So I conclude that I rarely put a force of more than 400 lbs on a part when it is being machined. I should mention the corresponding spindle load might be 80% or more so I am taking good cuts.
Drilling can put much more force and I often push the servos over 100%, often drills are pushing the part down into the clamp or vise and things will move during drilling that will never move during milling.
Your point about force above the clamp is a good one; you need to consider leverage which can multiply to tool force many times down at the clamping point.