Putting IB rebates into your expectancy math properly

[FXS London Scalper]

Expectancy is average win multiplied by win rate, minus average loss multiplied by loss rate, after costs. Rebates reduce costs, so they belong in the calculation. The question is how.

Example: EURUSD scalps, 0.5 lots, $3.50 commission per lot per side, 0.2 pip average spread, IB rebate $2 per lot round turn.

Cost per trade without rebate:
- Spread: 0.2 pips x $5/pip at 0.5 lots = $1.00
- Commission: $3.50 x 2 sides x 0.5 = $3.50
- Total: $4.50, or 0.9 pips equivalent.

With rebate:
- Rebate: $2 x 0.5 = $1.00
- Net cost: $3.50, or 0.7 pips.

If your average winner is 4 pips and loser is 3.5 pips at a 55% win rate:
- Without rebate: (0.55 x 3.1) - (0.45 x 4.4) = 1.705 - 1.98 = -0.275 pips. Negative.
- With rebate: (0.55 x 3.3) - (0.45 x 4.2) = 1.815 - 1.89 = -0.075 pips. Still negative, but much closer.

The example shows the rebate can't fix a weak strategy, but it can make a marginal one viable. Worth calculating honestly with your own numbers.