Weighted Standard Deviations and Calculating RoR

Hi all,

I was hoping to get a spot check on some of the RoR analysis I did.
The assumption is that a team of blackjack players is playing using both a shoe game and call in game. The shoe game comprises 80% of time played vs. 20% from the call in game. I'm sure my inputs above the dotted line are correct, however everything below is suspect.

Risk of Ruin ($25 Unit)

Bank Roll $15,000
$/Shoe (Shoe) $4.21
$/Shoe (Call) $23.28
SD (Shoe) $155.98
SD (Call) $407.82
% Shoe 80%
% Call 20%
---------------------------
$/Shoe (Total) $8.02 (.8*$4.21 + .2*$23.28)
SD/Shoe (Total) $206.35 (.8*$155.98 + .2*$407.82)
Variance $42,579

Ruin 5.92%

Formulas used from Wong's site:
http://www.bj21.com/bj_reference/pages/overheadeffectonriskofruin.shtml

My question is this - Assuming the 80%/20% model of play, can I just take the weighted averages for $/Shoe and SD or is that faux pas?

Thanks!