Re: not your typical rowing question
Only you can judge your exertion levels. No one can answer this question for you.
Your best bet is too go do a couple of rows on a Concept 2 and get a baseline, then compare how you felt on the C2 to what you do on your machine and adjust accordingly.
OR..you could measure the stroke, estimate drift, get a number and figure out how many strokes to 100M, and count it out.