Knowing your comfy aerobic pace and your corresponding TIME on the 2×200 swims in each of the four rounds means you have a target for each set of 5x50s, always with first 3 at pace minus 3 seconds and the final 2 at pace minus 4 sec.
If each of your sets gets a little quicker, then the 50s should get a little quicker every time, too. Good luck.
Asheville Masters Swimming 