I've played with the online gearing calculators but the only thing that really worked was to just make the gearing changes. On my two old school 7-speed road bikes I've swapped around between the original 52/42 chainrings and various combinations of 50, 39 and 38 chainrings. And between a batch of 7-speed freewheels, from 13-24 to 14-28.
I'd try to ride each combo for about a week, or 2-3 rides of 20-30 miles with enough hills to use the complete gear range. But in some cases the combinations were so awful I'd change 'em immediately after returning home.
For now I'm satisfied with a 50/38 chainring and SunRace 13-28 freewheel on one bike, a 1980s Centurion Ironman. Also good was a 50/39 double and SunRace 13-25 freewheel. I couldn't get along with the Suntour Alpha 13-24 and 13-26 freewheels, so those are back in a box for a maybe-future project. Besides the inappropriate gear steps, they ran noisily with clunky shifting.
The other bike, an early '90s Trek 5900, has 52/42 Biopace with 14-28 Shimano freewheel, with even numbered steps throughout the range so each shift is practically seamless -- very nice for shifting on climbs. I've tried that bike and chainring combo with 13-25 and 13-28 SunRace and the 14-28 Shimano seems about right. Just minor differences in gear steps, but enough to make a difference on climbs when I'm close to exhausted. The last thing we want on a hot summer day when we're on that umpteenth endless hill with false flats is a jarring gear shift taking us out of our rhythm.
So for me it's mostly try 'em and swap 'em until I'm happy.