Markup and margin are not the same number
Markup is measured against what the job costs you. Margin is measured against what you charge. The same job produces two different percentages, and the bigger the number, the further apart they get. A 20% markup is a 16.7% margin, but a 100% markup is only a 50% margin. Same money, two ways of saying it.
Which two numbers you start from matters, and it is the reason this tool asks. Cost and price together give you both percentages. Cost with either percentage gives you the price. Price with either percentage gives you the cost. Markup and margin together give you neither, because they are the same ratio written twice: no pair of percentages can tell you what anything costs.
Read the money-left-over figure, not just the percentage. That amount is what carries your overhead, your own pay and every hour that never made it onto the invoice. A percentage that sounds healthy on its own can be a thin number once the job is finished.
The mistake this exposes is quoting at a markup while believing it is the margin. A supplier gives you a cost, you add your usual percentage, and the margin you actually kept is smaller than the number in your head, on every job, quietly. It stops being guesswork when the cost and the price both live on the line item, which is what the Zeus price book does.
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