We use the same rules as in the Speccy Tour 2004. You are only allowed to play 150 Colony in-game minutes, not real time. This is to avoid huge recordings. So when the game says 02:30:00, stop recording. We then count the score like this:
Credits / 10 + whole Colony minutes played
ex. You earn 5200 credits, and plays for 01:21:34, then it would be this:
5200 / 10 + 81 = 601
Colony Regulation Play calculation help: