Similar to a default margin for new ad-hoc items, have a toggle in Profit Settings that allows Estimates to bulk apply a specific default margin. When an Estimate loads for the first time, all items would have a default margin set, despite what is stored as a markup/price in Products & Services.
This is especially important for organizations that use the same margin for all items. This way if a business margin changes, we don't need to modify every single item's markup/price. We could just have a override setting in Profit Settings that always applies 40% margin to all items on an estimate (unless its locked).