Generally speaking 15 minutes are flavor 0-5 minutes are aroma and 15-whenever are bittering.
The alpha acids isomerize during the boil at a generally predictable rate. The flavor compounds (phenols) are not all that water soluable and tend to volatilize and vaporize and leave the wort for a better place much quicker that the humulones isomerize. A rule of thumb is to use the above numbers.
Additionally, consider removing your hop additions at flameout (unless using flameout / whirlpool hops). Also consider that flameout/whirlpool hops will isomerize at close to the full rate depending on time and temperature. An old idea that 5% utilization could be expected is not correct. If you take 60 minutes to chill to xfter temp then you can expect upwards of 60-70% utilization.
This means that if your beers are more bitter than you like, remove the hops at flameout and cut back on the flameout/whirlpool hops.
- Joe