Depends on how far apart the steps are. On a 21 step wedge they are usually 0.5 stops apart. So that would be 0.15 log density change between each step (recall that one stop is 0.3 log, so half of that is 0.15 log density.

The ISO(R) values are computed as 100 times the log density the paper will reproduce.

So, if you get 6 gray strips, that is 6 x 0.15 x 100 = 90

These are rough estimates, but the ISO(R) values are ranges anyway.