Marketing efficiency ratio is total revenue divided by total ad spend. This works out the MER you need to break even from your contribution margin, compares it to what you're actually running, and tells you how much spend headroom is left.
Enter revenue above zero to get a result.
Payment fees, fulfilment, shipping you absorb, returns and refunds.
Costs reach 100% of revenue — there's no margin left to spend against.
actual MER, against a breakeven of 2.00x