what they do is they work out your percentage (of marks you were awarded) e.g. 64% and then they take that away from 100 to work out the percentage of marks you didn't get, so 64% would be 36pens.
i'm pretty sure that's right - but i know they do it differently in 3 day eventing.
well i'm afraid it does get slightly more confusing as you get to 3 day eventing (not sure exactly which level) because then they do multiply it by a small number (you may be right with 0.3!)
but if it's just for something like intro and PN, then it's easy