Find your real margin
Work out your real margin first: price minus materials, packing, shipping and platform fees. Everything else builds on that number.
Think about the extra sale
Then decide how much of that margin a sale you wouldn't otherwise have made is worth to you. Every sale through another store is one you didn't have to find.
Adjust with real numbers
Review the rate after your first sales. It's easier to adjust with real numbers than to guess at the start.
Watch two signals: whether stores add your product, and whether you're still happy with what's left after each sale.
A worked example
Take a €60 product that costs €30 to make, pack and post, leaving €30. At 12%, you pay €7.20 and keep €22.80 on a sale you didn't have to find.
At 30%, you'd pay €18 and keep €12. That might suit a product you want in front of more buyers quickly, and not one that's already selling well.
Check it from the store's side
Stores compare what they could earn across products. A rate that's fair for a €10 item may not be worth a store's attention. Lower-priced products usually need a higher percentage to stand out.