It's not how far forward, but rather how far back the flashes should go. Imagine a line along side your leg from your ankle bone to the top of your hose. Your flashes should go no farther back on your leg than to the point where the line bisects the center of the flashes. If you prefer your flashes a little more to the front, then the back edge of the flashes should just touch that imaginary line.

Generally it is considered appropriate to have 2-3 inches of flashes showing beneath the cuff of the hose.