Barry, I have a Vasa and substitute my swim workouts as written, like I’m in the pool. At least with my machine, the Pace on the display is really close to my actual pace in the pool, so I use my pace zones just like in the water. Let’s say there is a 5×200 meter repeat on 4:0, I’ll do the 200 in 3:00, then rest just like I would in the pool.
I do make a couple of small changes. I’ll switch to “fly” for 25% of the time, just to break up the routine. I’ll also switch the paddles to the grips, sit up and face away from the flywheel and “swim” with the resistance in the opposite direction. This is especially good to prep my shoulders for the resistance introduced with wetsuit swimming.
David