Doc MillerOpen Toe Compression Socks, 30-40 mmHg, Toeless, Support Circulation, Medical Grade, Socks, Men & Women, Pair

Is this truly 30-40 as the sizing chart says 20-30.

Under what size should i get it says this: delivers true 30 to 40 mmhg compression, so wearing them may take some effort? what's this mean?

Is this the best Compression Socks brand? The quality of how?

How do you know what size to buy when there's not a size chart?