Right now there is a big problem with refback offers because affiliate statistic doesn't show you your referral commission but only your referral's current balance that means almost nothing (it becomes zero every time your referral withdraw money).
You cannot calculate total earning from this particular referral.
I calculated also the refback. My brother has also a paid account and i got the refback for this  
We tested it. Directly after he pressed the arrow to move the coins to the balance, i always got the correct percents. At the moment 35%.