"Low gearing: Lower the better. Smallest inner chainring possible. Biggest rear cassette cog possible. Your crankset seems to be 74mm bcd inner. So 24 is the smallest. Mountain bikes seem to use 22 teeth as the smallest. Not much difference."
A difference that can make a difference. Not much difference in number of teeth, significant difference in percentage, 8 percent. Would you say dropping from 52 teeth to 48 teeth for large chainwheel is not much difference? Same 8 percent.
As already stated, percentage changes lose their significance when you are talking about small gears. For small gears you need to look at the difference in gear inches between gears. Percentage difference is OK at the large gears. For instance, take gears of 1 inch and 2 inches. 50% or 100% difference. The 2 inch gear is TWICE as hard to pedal as the 1 inch gear. In reality, you could not tell the difference on a bike. Same logic applies for low bike gears around 20 gear inches. You can tell the difference between a 25, 20, and 15 gear inch gear. But in reality, there isn't much differene. All will work just about the same climbing a hill. It would be nice to have the 15, but the 20 is perfectly fine too. And the 25 does not put you at much of a disadvantage. All three gears would be equal enough. Huge percentage difference though.
In another post it was commented that STI would not reliably shift to a 22 chainring.
? My Shimano STI 5703 triple 105 shifters shift a 44-33-20 crankset perfectly well. STI shifts all cranksets fine. There can be difficulties in getting the front derailleur set up properly. But the STI shifter will shift between any and all chainrings fine. Size is irrelevant.