Why intraday friction dominates
Day trades have less time for a move to outrun the spread and commissions. A provider can show a profitable gross backtest while a subscriber using a different broker loses the edge. Enter a full round trip: entry and exit spread, commissions, slippage and any financing charge applicable to the holding period.
Measure R on the original stop
R is the cash risk between planned entry and the initial protective stop. If risk is 100 currency units, a 0.1R trading cost is 10 units. Calculate cost from actual fill records, then run the worksheet for the median and adverse-cost sessions. Do not use the provider's chart price as a substitute for executable quotes.
The numerator still matters
The result tells you which win rate would break even given average winner and loser sizes. It does not tell you whether the service achieves that rate. Require a complete time-stamped sample, count same-session missed calls and inspect tail losses. A tiny sample is particularly fragile when a few large winners carry the average.
Evidence and editorial standards at Best Day Trading Signals