Frame seat tube length centre/top ( horizontal top tube ) is close as damnit to 2/3 leg length. Leg length is your standing height less your seated height ( not the book method, its too variable ).
Seat tube angle is from a formula which involves the lower and upper leg measurements. When this is correct and the saddle is mounted at the manufacturer's mid point of adjustment, the very front of the knee will probably be 'over the pedal spindle'.
Observations by lesser bicycle fitting people noticed this resultant coincidence, and adopted it as a 'short cut' to getting the fore-aft position correct.
Saddles have an adjustment, of course, because a cyclist might purchase a stock '74 deg Parallel' frame. The seat tube angle will more than likely be incorrect for his leg measurements.
Bike manufacturers like Specialized researched the average body dimensions of men, drew up a range of frames to suit the AVERAGE man of heights of 5 cm increments. It just so happens Spesh's angles and reach suit me perfectly, but maybe not the next man.
Burrows' idea of a Small, Medium or Large frame for everyone was advantageous for Giant insomuchas they could vastly reduce their manufacturing costs, and increase the sale price through calling it 'compact geometry' and effectively con the bicycle buying public with ill fitting frames, relying on saddle adjustment and stem length to get a reasonable fit. The ONCE team were PAID to use and ride the bikes. Free kit, a good saving off the budget.
In a recent edition of CTC's magazine, Burrows recommends putting your forearm between the saddle nose and the handlebars to get the reach correct. We all know this is codswallop because saddles vary in nose length from maker to maker.
Reach is determined by many other body and arm dimensions and should result in a lumbar angle close to 45 degs when riding on the hoods.
Seat height ( BB centre to top surface of saddle ) has been described as Leg length x 0.883. That's OK if the crank length is correct for your hip height. If the crank length is 2.5mm out, that won't be too drastic and I doubt if a new rider will notice the difference. If the cranks are 5mm out, it might be noticed by a very experienced cyclist.
Yet again, my Spesh is 'cock-on', and Leg length x 0.883 works good.
500 km in two days proved this OK for me.