Both posts give good advice.
The reason you don't need a definitive amount of exposure for a step wedge on film...
An ideal exposure would give you the entire toe of the film curve. Worst case you overexpose by several stops and even the darkest step wedge patch makes a density over 0.10 on your test film.
Then you only need to count out how many steps you wish you had in the toe... and give that many steps less exposure when you make the next test.
You are exposing a test pattern that covers 10 f/stops worth of range. If you overexpose or underexpose by several f/stops, you still will get a test pattern on film that will tell you how far off you are in terms you can easily convert to f/stops.
Being off by 1/3 stop (which might be noticeable if you were shooting transparency film in camera), has negligible impact on a film test. To be off, you have to be several f/stops off. And the necessary correction is easy to quantify.
Now for Consistency sake, it would be nice to be within 1/3 stop of repeatability... but as far as the landing point, you do not need accurate placement of the test exposure.