Swimming training zones

I tried to google around to find a reference on swimming training zones but I couldn't.
Looking at the Friel bible, there is a table for swimming zones but it is not clear to me how he is calculating it. In general, for running for example, you can apply a % for each zone and you will have the speed relatively to 10k PB (or 5K).
Now, for swimming it seems that for the best category, he applies a different % than for slower swimmers.

I tried to make my own claculations which can be apply probably to everyone.

Zone 1: 110% +
Zone 2: 110% - 104%
Zone 3: 104% - 96%
Zone 4: 95% - 92%
Zone 5a: 92% - 87%
Zone 5b: 87% - 81%
Zone 5c: 81% - Max

The % is relative to your 100m pace over 1k swim test.
Example: if you recorded 20'00" on 1k, you have 2'00 pace as reference and zone 1 is 2'12 per 100m or slower.

I think it is working well for me with something like 20'00" on 1k test but for better swimmer I don't know.

Do you have a better reference? or any comment on my table?