Identify your use: caipirinha-style (lime + sugar), a rum-forward cocktail, or a recipe that mainly needs alcohol strength.
Pick the substitute that matches the flavor job: light rum for clean cane-like sweetness, gold rum for warm notes, vodka for neutrality.
Use a 1:1 ratio by volume (same amount of substitute as cachaca).
Adjust after mixing: if the swap tastes flat, add a small amount of sugar; if it tastes too harsh, add a touch more lime to sharpen.
Taste once the ice melts slightly, then fine-tune with one adjustment at a time.