In flight I use quick calculation rules that are easy to apply and 98% accurate or better. It can literally be quicker to be running an e6b for cross country preflight calcs than doing it by hand…but now software does that, from private pilots to 121 ops. The rules of thumb always apply though and can be used in flight, also logistically way easier than busting out an e6b or calculator. That is what I do while flying, run my own numbers and compare to the FMS/gps/cdi etc.