I'd like to know how much percentage of sP he takes when the random event appears, so any help would be awesome. That way I know how much I need to have to at least have a chance on the 500k at once achievement :')

He took 5% from my friend when he hit her yesterday. Not sure if it's always that, or if it's sometimes more or less, but yeah.

he randomly takes between 1 and 5% making it very difficult to judge how much cash to have. You can base it on 1%, but then if he happens to take 5%, you lose 5 times more than you need to. On the other hand, he visits so seldom (for me anyway) that you might want to lose more so you can be done with the achievement.

Thanks both for the help! I just noticed the achievements go up to like a total taxes of 100m, so I think I'll have all my sP on hand and deposit it at the end of the day for the interests. This will take a while though.