A 2% pre-mine in which one block of a coin that mints 100 coins per block would be two coins pre-mined.
If all 100 of the coins in that block were pre-mined that would be a 100% pre-mine.
So for example if the code forces the first block to have 102 coins instead of the normal 100 coins, that would be a 2% pre-mine.
If the code forces 10100 coins to come into existence in the first block instead of the normal 100 coins on a 100 coins per block coin, that is a 10,000% pre-mine...
Sure after a nubmer of years that might dilute down so much it will by the time no more coins are beign minted appear in retrospect to be a smaller percent but the future doesn't exist, there is only the now...
Another way to look at it is to check how many blocks had been mined 24 hours after the launch.
We know how many blocks the coin pretends it is supposed to have per minoute or hour or day, so any more than one day's worth of blocks beign in existence at the end of 24 hours indicates how many block-days (days as measured in blocks, given the block time the coin pretends to have) of pre-mining occured by the time 24 hours had passed since launch.
Bitcoin is pre-mining a few days every two weeks right now by that method of measurement!
When bitcoin began it was not pre-mined, but as GPUs then FPGAs and now ASICs came along more and more retroactive pre-mining has been happening!
(In case you don't get it, multiply ten minutes by the block number. To get to this block number in that amount of time when would Satoshi have had to have started mining?)
-MarkM-
