I recently learned that good, safe squat form could change based on body proportions, specifically the factor femur/spine length ratio. How exactly does one measure them both (from where to where, I mean) and how should squat form (width of stance and angle of feet, mainly) change based on these proportions?
Caveat: Proportions is an indicator of how your squat form could look. Actually measuring your limb and torso lengths and trying to mathematically deduce "correct" form is rarely beneficial, and can lead to someone trying too hard with a form that simply doesn't work for them.
This is partly because no one has a handle on all of the significant factors that will influence your squat form. For example, I have very long femurs and should theoretically do well with a wider stance allowing me to stay more upright and minimise back involvement. In real life however, a moderate stance allows me to generate the most power.
With that out of the way, for the purpose of experimentation, I find that this article provides some solid information and general guidelines on methods of squatting depending on your body proportions:
...the femur averages 26% of the body’s total height, so measure from your hip to the outside of your knee joint. Divide that number in inches or centimeters by your height in inches or centimeters. If you get .26 on the dot, then you’re average. If you get more than .26, you’re erring farther towards “long femured” and if you get less than .26, you’re erring farther towards “long torsoed.”
Then follow the guidelines on what you should do if long/short femured/torsoed.
Again, use such recommendations as a starting point, but be sure to experiment and find out what works for you and allows you to feel strong and squat to depth.
I should also mention that your "best" squat form can and often does change over time as you train.