With a 21 step wedge you can do a quick calculation without a reflection densitometer. Throw out the one that is "almost black" and the one that is "almost white." Count the number of slices between the two. Multiply this by 15 and you have the ISO(R).

Typical figures might be Grade 5 = ISO(R) 40 to 45; Grade 4 = ISO(R) 60 to 70; Grade 3 = ISO(R) 80 to 90; Grade 2 = ISO(R) 100 to 110; Grade 1 = ISO(R) 120 to 130; Grade 0 = ISO(R) 140 to 150; Grade 00 = ISO(R) 160 to 180.