Decide what matters most: peppery bite (choose turnip/daikon) or pure crunch (choose jicama).
Match the quantity using the listed ratio; for salads, slice/shred the substitute as thin as you would the radish.
If the substitute is milder than radish, add a tiny pinch of salt or a squeeze of lemon to bring the flavor forward.
If the substitute is watery (like cucumber), pat dry and add it at the last minute to avoid soggy salad.
Taste and adjust intensity: add extra vinegar or a few grinds of black pepper to replace radish’s bite.