If you take the time to use a fit calculator it will get you close and you will have more time to try close guesses then dial them in than guessing and going.
Established ratios and proportions exist because they work. You're not inventing anything, you're adjusting a bike and it's been done many times before with great success. If you follow existing guidelines to get close it will make things a lot easier. You're not likely to score any goals swinging at a tetherball with a football bat.
This is an accepted quality fit calculator.
Get a helper to assist in taking measurements. Measure three times and use the average, most find metric easier to work with (plus they translate to bikes easy). Plug your numbers into the fit calculator and adjust your bike accordingly. You should be very close at that point.
