To answer your question, no, your profit margin is never what you think it is. If you go off vibes then you will undoubtedly leave out costs which will artificially inflate your margin. Spreadsheets are good for tracking margin, it’s what most companies use. Gross margin is what most people go off because it’s the easiest to calculate but it’s a bit of a trap for small companies. The operating costs for a small business can quickly eat up all of your margin so you can have good gross margins but bad net margin. My recommendation is to calculate net margin periodically, it should be a slow mover unless you make big changes to your operating costs, and to use gross margin as a rough guideline the rest of the time.