Ultimately, it’s going to be trial and error. As your strip is relatively short, the cars won’t have quite the same momentum through the finish line as with a longer strip - and they should stop sooner. Five feet of shutdown after 11 feet of powered running seems a good place to start experimenting.
For my HO strip, I cross-wired the rails after a short dead section to create a dynamic braking circuit in the shutdown. I don’t know if that would risk damage with the sort of motors you’ll be using? You certainly want to use some heavy gauge wire under the track and maybe introduce some resistors or diodes into the circuit to reduce the braking effect. A 1/24 drag strip owner would give better advice. The amount of braking effect will vary depending on the motor being used, so that’s another factor to throw in to the trial-and-error process.
An HO shutdown benefits from friction of the cars’ traction magnets - I’m not sure if there’s a safe way to add an element of friction to a 1/24 car. However, you could use gravity to help slow the cars, by adding an upward slope to the shutdown. It won’t have too much effect, but might shave a couple of inches off the required shutdown length.
You will absolutely, definitely need a well-padded catch box at the end, just in case things go wrong