"Umm, leg press is a compound movement.Hips and knees, the ankles don't count."

  Pre-exhaustion techniques vary by individual. I can't tell you how much weight to use or even accurately predict a percentage. Many people tell you that there is a percentage, but that theory contains too many flaws. The biggest flaw would be the predominance of type I or type II muscle fibers. These are white (fast twitch) and red (slow twitch) fibers.

Red muscle fiber carries oxygen better than white fiber whereas white fibers are better for speed and power, but fatigue much sooner due to lactic acid buildup. Lack of tissue oxygenation is what causes lactic acid to build up (lactic acid isn't what makes you sore either).

Basic rule of thumb is to use a weight that you can complete for ten reps to failure. There isn't much point to pre-exhaustion if you're not training to failure.

So leg extensions for ten reps of what you can complete for ten reps followed by leg presses with about 50-60% of your one rep max (1 RM).

Other pre-exhaust techniques:

Flyes into presses, whether they're inclined or flat.

Side lateral raises into shoulder press.

Bent side lateral raise into rows.

Narrow grip curls into wide grip (it works).

Press downs into close grip bench press (BE CAREFUL).

You get the idea. The fact is everybody is built a little differently so most of this is trial and error. Concentrate on sets of ten reps using the above guidelines.

If you really want to make it ridiculous then try doing the exercise that you started with after the compound movement again, but with less than half of the weight of your first set. This is known as a giant set and it's incredibly effective for boosting growth factors and other androgenic benefits.