"use $80 because this strategy relies on market orders so a lost tick on entry and exit plus $5 for comm. and you've lost $30 of potential profit. "
um, no.
classic mistake but no
look at a contract that has a spread. if you
buy the ask, sell the bid, you lose the spread.
you do NOT lose the "spread twice" as many people incorrectly assume
otoh, if you buy the bid, sell the ask (play market maker) you MAKE the spread (not applicable here).
if you buy the ask, and sell the ask, you neither make nor lose the spread. that's a breakeven (absent commissions).
so, assuming market orders on a liquid contract (you buy the ask, sell the bid), you lose the spread. that's $12.50 not $25
.
if it is a losing trade, and you are using a stop (not stop-limit) and it's moving quickly, you may lose MORE than the spread of course.
but, contrary to common belief, you do not pay the spread twice when you buy ask and sell bid. you pay the spread