I agree with Andy. Its very impractical to mount one on the thigh shround. Always put your reloads on the non gun side Belt or chest.
+1
I prefer to have all "primary use" mags to be on the weak-hand side. "reserve" mags can be located on your strong side but these aren't for use in the thick. You move them over when you have time.
Think of your body as having two halves: strong and weak...
STRONG SIDE GEAR
Sidearm
Reserve Mags
IFAK
WEAK SIDE GEAR
Primary Mags
Water
It balances out, and positions stuff where it needs to be.