Your best bet to get the correct carbonation is to set it at 12-14 psi for a week or two and serve it at the same pressure. The high pressure for a few days is very inconsistent and easily to lead to over carbonation. If you're going to shake it at high psi be sure you've burp the keg a few...
You're better off going too slow than too fast. Keep good notes of everything you do for comparison. Be sure to record a pre boil gravity reading. My fly sparge is generally around an hour long.