Not an expert but the math looks right.

20 block skill - 6 (30% of 20) = 14

Or...

20 block skill - death1 - 19.0 (-5% or 95% or original)
19 - death2 - 18.05 (-5% or 95% of 19)
18.05 - death3 - 17.15
17.15 - death4 - 16.29
16.29 - death5 - 15.48
15.48 - death6 - 14.7

Run skill 50 - 7.5 (15% of 50) = 42.5 for your friend. If your friend lost count and actually died 4 times the math is perfect (50 - 10 = 40).

That said, I think the "death penalty" is a bit of the steep side.