Your calculator is the best that I know of.

If you always use econ (COST 0) on your FMC, watch closely what your burn is in climb, cruise, and descent.  your burn will be similar (but not always the same) depending on your weight and flight level.

You can then make basic approximations like "I'll climb about average 2200 fpm to FL360" so to get from sea level to 360 should maybe take about 16 minutes, so if that burn is about 5000 pph per engine then this is 166.666 pounds per minute for both engines, so your climb might be near 2666.656 pounds for a 16 minute climb in this configuration.  And so on.

The numbers are just examples.   And that idea doesn't of course factor the winds or air pressure, which are both factors in an accurate calculation.

I did what I mentioned before in my flight last night, with a 15,000 lbs reserve calculation, and had some more headwind than expected, thus my landing fuel was 13,800 lbs.  Still more than 1 hour reserve, and FSPassengers-X was happy about it.