The Sweet Marias instructions are fine, and a 20ish setting is a good place to start. Note though, that that advice goes along with a 2 -2 1/2 minute steep.
The more traditional story is to use a regular grind (that's where the 30 to 40 setting comes in), and a five minute steep.
The Sweet Marias procedure will give you a slightly hotter coffee with more sediment, the traditional procedure yields a slightly cooler brew with less sediment.
Sweet Marias says the finer grind yields a better extraction. I don't find this to be true or are least meaningful to me. Finer grinds extract faster but better? Not to my taste.
YMMV as to what grind/time/sediment/temperature outcome you like the best. Experiment away!
Trust your taste. Don't trust your perception.