The axle ratio that puts your engine where you want it as you cross the finish line. Use a real trap speed if you have one, or estimate it from horsepower and weight.
This is the same rpm formula as the gear ratio calculator, solved for the axle ratio. Most racers aim to cross the finish slightly past the engine's peak horsepower rpm, so it's pulling hard all the way through the traps.
This is Roger Huntington's classic estimate. For the eighth mile, trap speed is roughly the quarter-mile figure ÷ 1.26 and ET roughly ÷ 1.57, both common rules of thumb.