"20 seconds on" is pretty much useless on a rower. Either use a fixed number of max effort pulls, or just stick to 500s on the rower.
If you're decently strong and conditioned do 4 sub-1:40 500s with 2-3 minutes rest between each. HARD.
Then aim to get strong & fit enough to get below 1:30 for 4. Super fucking hard.