How are you calculating it now?
I take the starting value of BTC->USD exchange rate, convert my remaining BTCs into USDs and then do the regular ROI calculation.
startUsdValue = startBtcs*exchangeRate + startUsds
currentUsdValue = currentBtcs*exchangeRate + currentUsds
ROI = 100*(currentUsdValue - startUsdValue) / startUsdValue
But, it seems wrong somehow - just wanted to make sure there wasn't another more correct way?