A lay-to-back trade lays a horse and aims to back it at a longer price once it drifts. The calculator uses the real ladder to find the target price, the back stake that levels the book, and the profit.
Worked example
Lay £20 at 4.0 (liability £60), target 3 ticks. Between 4.0 and 6.0 a tick is 0.1, so the target is 4.3. Back stake = 20 × 4.0 ÷ 4.3 = £18.60, green +£1.40 (+£1.33 after commission).
While the lay is open your exposure is the full liability, so a horse that shortens instead of drifting can cost far more than the stake. Size the trade on the liability, not the stake.
Questions
Is lay-to-back just back-to-lay reversed?
The maths mirrors it, but the risk profile differs: the open position risks the liability rather than the stake.