Base your budget on the money one customer brings, not on “what you don't mind losing”.
Work out how much you can pay per lead
Take your average order value, multiply it by your margin and by the share of leads that turn into sales. That's the maximum cost per lead at which ads break even.
For example: a €120 order, 40% margin, and one in four leads buys. 120 × 0.4 × 0.25 = €12 per lead. You can run your own numbers in the calculator on the home page.
Set aside a test budget
In the first 1–2 weeks the ads learn and you test creatives and audiences. The budget should be enough for each variant to get its first leads. If it's small, test fewer variants at once.
Scale gradually
Once the cost per lead is stable and below your limit, raise the budget gradually so you don't disrupt the campaign's learning, and watch how the cost changes.