Work backwards
Gross profit per order is the absolute ceiling — spend that and you break even. Decide what share of it you are willing to hand to Google, and that is your target.
Example
A €70 order with €28 gross profit. Break even at €28 CPA. Keep a third, and your working target is roughly €19.
What people do instead
Pick a number that feels affordable, discover it is unachievable, loosen it repeatedly, and end up with no target at all. Or set it so aggressively the campaign stops spending and looks broken.