Where do you do back of envelope calculations?

Before Apple killed the Dashboard in Catalina. I had this widget called FormulaCalc that I loved.
It was very basic, screenshot below. It had two long text fields on top of each other. You'd enter a mathematical expression in JS in the top field (e.g. "3 + 4 / ( 5 * 0.9 + pi) + pow(2, 4)" ) and as you typed it would auto-generate the result in the bottom field in real time. I would have several instances of this widget in my dashboard so I could quickly and easily do simple calculations or do multipart calculations, copying the result from one widget into the input field of another.

I'd use it constantly for things like measuring stuff for frontend work, doing some rough analytics from db queries, or even just back of envelope budgeting / tax stuff. Using it was so automatic and immediate that it became such an essential tool.

Screenshot: http://www.mindlessflash.com/formula_calc.png

Since I upgraded my OS I've been using the evaluator in Spotlight and the calculator app on my phone and it's not as good because you can only work with one formula at a time. So I guess I have two questions:

1. How do you do rough calculations day to day, what tools do you use and are you happy with them?

2. Do you know of any products / solutions that are similar to the dashboard widget I described above?