It helps to use some reference point on the bike and I use the bottom bracket.
Many tall riders use cranks of M length. To get a good position (some varient of K.O.P.S) they have to move the saddle back or get a really slack seat-tube angle.
If you select a proportional L crank you foot will be further forward from the BB at 3:00 so your saddle will be equally forward (ie a normal distance). Your weight will no be so far over the rear hub but more balanced.
This will allow you to use a longer stem for the same reach, which for a big rider is usually >120mm .
By selecting proportional cranks and designing the frame
around them you solve all the problems of bodged geometry and unbalanced weight that most big frames create.
Note how it is really difficult to judge the size of AnthonyG's bike, it is tiny but perfectly proportioned, built around proportional very short cranks.