For what it's worth, here's my opion: A deer that is going to develop into a mainframe 8 and definitely a mainframe 10, is going to be a mainframe 8 at 2.5yrs. Therefore, you really won't be protecting them by putting that regulation out there if the guys you hunt with are simply looking to kill any buck they can.
So what regulation does let them grow? I honestly don't know. I hate the spread one. I have three deer just shy of 125 at my house that won't stretch the tape more than 14"s inside. Main beam length seems to be the best answer to me. 18-19" would be a good place to start, but that is tough for a hunter to judge, especially if they are just looking to kill a buck.
I do applaud your efforts of trying to let the bucks get age on them. Hopefully, you can develop a plan and get everyone to agree and follow it.