It's all about ratios. Ratios doesn't change whether you have freewheels or freehubs. You could use a site that talked about cogs for a steam engine and it would be the same thing if you could put in the same cog tooth numbers.

Edited to add:
I have a belt drive, not a chain, yet it doesn't matter in those calculators because it is the ratio that matters.

I was trained early on to use the gear-inch lookup tables. I learned to think in gear inches. For fixed gear bikes, gear inches are an excellent comparative measure for the result of your chainring-cog combination.

Any bike for which I'm selecting and installing the drivetrain, I ALWAYS go through several iterations of the gear inch calculator to make certain that I'm getting the gears that I need and not making gear combinations that I neither want or need. I have even printed out the gear inch table and taped it to my stem top for display (this for a half-step plus granny set up). But these are simply numbers.

I'm building up my '71 Jeunet right now, using a Campy chainset with 116mm BCD rings. They're not common and not cheap, so selection of gear ratios is particularly important. I'll have a 14-24 block in the rear. That's it. The good news is that I've found that for everyday riding, I need nothing lower than 40 G-I and really no higher than ~90 G-I.

Gear inches are huge in the BMX racing scene. By huge, I mean that there’s a high awareness of the numbers, not the numbers themselves. (Most competitive folks run around 55 gear inches.) Precise tire circumference measurements are also a thing, since a tire change can easily have a greater effect on gear inches than going up or down a tooth on the chainring. But racers tend to say “roll out” instead of “gear inches” and many seem to think they’re going 55 inches per revolution of the cranks, not 55 * π. If you do the math, you’ll find people are revving up to 150~250 rpm to hit 25~40 mph on 55gi.


https://cimg9.ibsrv.net/gimg/bikefor...f7eb080a1.jpeg
Vintage gear chart photographed at a San Diego County race track in 2018. Note the inclusion of sew-ups! Modern charts will be broken down by different tire models, e.g., Tioga’s 20 x 1.75” models are much larger than Maxxis’ tires at the same nominal size.

For now.

Gear Inches is my lingua franca in dealing with the unsuitable (to me, as I am a masher) gearing that is common on newer small wheel bikes.

And if the advertiser has the wheel, crank and cassette sizes then I know what I'm getting. And it's usually preposterously low. Probably good if you live in San Francisco.

I've upgrades the gearing on my Mercier Nano and Downtube 9FS by upgrading the cassette and increasing the chainring size.

BTW, any old 'euro' folder (MIFA, UNIS, ROG, Italian) usually was around 60 gear inches.

Before bike computers, calculating your speed on the fly by knowing your gear inches and cadence (and wheel size of course) made for good mental arithmetic on those long boring rides. Just don't slow down.