Decide what you need green onion for: garnish-freshness, mild onion bite in cooking, or color/texture.
Choose the substitute that matches that role from the list, then use the stated ratio as your starting point.
If the substitute is sharper (onion), use thinner slices and/or soak briefly in cold water to reduce bite.
Add most substitutes early if they’re chopped small, but save chives/garlic chives for the end if you want maximum freshness.
Taste and fine-tune: add a pinch of salt or a drop of vinegar/lemon to brighten if the swap tastes flat.