dude, Roy.... ppl are trying to help you out here... maybe you should listen instead of ignoring advice you don't want to hear...
Where did your magic number "start the safety stop at 500psi" come from? What does it really mean? what happens if, at that exact moment your buddy goes OOG? What if you get caught up in some kelp and it takes you longer than intended to get out? what if.....
I strongly strongly advise that you do some reading on gas management strategies. There are probably several ways to slice this pie, but at the you should at least have some understanding of why.... I would suggest you PM NWGratefulDiver for his gas mgmt handout, or look over lamont's writeup of "RockBottom". Even if you don't decide to use it, it might give you a better understanding what you are doing.
BTW, gauge inaccuracies aside, a typical intermediate pressure for SCUBA regulators is about 140psi... so 150psi really is ~= 0.
Just because you can do something once, twice or several times without problems, doesn't mean it's not inherently flawed or dangerous. # of dives is not necessarily a great indicator of experience either. Doing something poorly many times and getting away with it is really no better (in fact worse) than doing something well few times. If I could go back and talk to myself at 100 or 150 dives, I'd slap myself upside the head. I'll probably say the same thing another 200 dives down the road. Next to alot of the guys around here, I might as well have learned to dive yesterday.
I'm sorry, but to be perfectly frank, but not wanting to "waste" part of your $10 nitrox fill is just plain stupid... It's not a contest to see who can use the most gas. Next time you want to go on a boat, hit the BW or Lobos instead, and you just bought yourself an extra 500psi in the next 54 tanks of nitrox.
Perhaps a more productive (and safe) way to approach your goal (more time underwater) would be to work on improving your SAC rate while maintaining adequate reserves?
I usually don't like to throw advice like this around, especially not with this tone, but really, people are just trying to look out for you, so take from it what you will.