You don't add commissions, you minus that off the credit.
$20 - $6 = $14 net.
Max loss = diff between strike - initial credit received
= $100 - $20 (not net here or you would be double counting commission)
$14/$80 = 17.5%
Assuming you use max loss as margin, and calculating return on margin. In case of loss, you have to incur another round of commission, assuming you didn't count it in $6. So it changes there again. If you already factored round trip but end up profiting expiring worthless, it changes again to higher ROI.
The following user says Thank You to k20a for this post:
Got it, thanks for the clarification. Yes, the $6 would be total commissions on TOS ($1.50 per contract). I've been setting trades to close at 70% of max profit, so I count on having to pay to close out.