Ask ten Amazon sellers how they set their daily PPC budget and most will give you a number that came from instinct rather than math. That works fine at low volume. It stops working the moment a campaign starts scaling, because ad spend that felt reasonable at ten orders a day can quietly become unprofitable at one hundred.
A better approach ties budget directly to your break-even ACOS. Calculate the maximum cost-per-click you can afford at your current margin, then let the campaign spend up to that ceiling automatically rather than capping total daily spend arbitrarily. This keeps every dollar spent inside a profitable range instead of rationing spend on a good day and overspending on a slow one.
From there, separate campaigns by intent. Branded and exact-match campaigns targeting proven converters can run with a higher acceptable ACOS because they are defending market share you already have. Broad discovery campaigns should run on a tighter leash and get reviewed weekly, since that is where wasted spend accumulates fastest.
Scaling PPC well is less about spending more and more about spending in the right place at the right ceiling. Get the framework right once and the budget question mostly answers itself as the account grows.