Call the very day you reach MPG minus 4:30. Otherwise you may miss out on the pay bump. If you get within 4:30 and spend another day on reserve without calling, you missed out on 4:30 getting added to your line value. Or, you get within 4:30 and you fly less than 4:30, you just lost the difference in pay. As soon as you get within 4:30, call.

P.S., they will also "level you" when you're getting close to the MPG minus 4:30, which is to say they'll cease flying you to prevent you from getting the extra pay. They won't fly you but they won't release you either. You can sometimes trick the system by personal dropping a reserve day that is over coverage (I know, very rare) which lowers your MPG thereby bringing you within the 4:30 from the top down vice from the bottom up. If you have at least one day left on reserve after doing this, then the value of the day you just dropped gets added right back in. You just went from 12 days off to 13 days off--with the same pay. I did this for many months, getting one (or more) extra days off and still getting paid nearly what my original MPG was.