This is pretty much the worst possible way to adjust a prop valve, and is also a piss poor assessment of braking performance.
If it won't lock the FRONTS you're losing stopping ability. If it EVER locks the rears, you're spinning out of control whether you like it or not.
There's so much more to this equation, it's not even funny. What condition are the rear shocks in? If they're blown, your car will have insane stopping power due to the weight transfer. My neon used to stop so hard it would literally make you light headed and nauseous. It also bounced down the freeway. New rear struts, and now it locks the fronts, predictably and calmly.
Riding the edge between stopping and slipping the fronts is called threshold braking. It's what ABS does, and it's what you should learn to do, because it's the maximum stopping power: there's no more grip to be had, but you're not sliding. Anything else, and you're giving up braking performance and that might be the foot or two you needed to avoid a crash.
Road condition, tire age and type (as mentioned), existing weight bias (full tank or running on fumes), brake fade, even the presence of a passenger or four, all count in the braking equation. A good hard stop without locking the rears is NOT the determination of a prop valve adjustment, nor is backing it off a couple turns to call it good.
Begin at 30 mph on a DESERTED highway that's lightly slicked with rain. You should be able to lock the fronts and have NO indication of rear lockup. Up your speed slightly, rinse, repeat. If your tires are worn or older, that's in your favor for this test. Adjust to get the rears to lock and back off for the max performance stops, but add in a generous safety factor to prevent rear lockup.
You should be able to lock the fronts without the rears ever locking. If the rears lock, you WILL spin.
Once the fronts lock without the rears, THEN you can learn threshold braking. If you ever change anything in the system, even just new pads, it's best to verify these tests again.
