1) Ending up .5g lower will give you a higher OG, higher FG, more IBUs, darker color. And sadly, .5g less in your final yield.
2) I used to have a very vigorously rolling boil and had a lot of evaporation. You don't need a rip-roaring boil. Just beyond simmering is all you need. Typical evaporation rates are 8-15%. Next time scale back on your heat until you have a nice calm boil. If the surface is rolling at all, it's boiling just fine.
BTW, there's nothing wrong with your over-boiled beer. You likely created more melanoidins which will make it more malty, biscuity, or bready. It will still be tasty.