Roughly 60% of a stroke's power comes from the legs. Common mistakes include "shooting the butt," where the seat moves but the handle does not, wasting leg power.
Use the erg for fitness and testing. Use the water for skill. A great erg score does not guarantee a fast boat. Training for Rowing
This assumes 8-10 hours of training per week. Adjust volume for your level. Roughly 60% of a stroke's power comes from the legs