Hi, I’m using a Garmin Forerunner 935 in a 25m pool. I did a 400/200 CV test the other day and the data added 2 lengths of 25m at 55s and 50s. How, I know I swam 400m (I learned to count age 3) and sadly, I know I can’t swim at 55s/100m without a jet ski.
From experience with two garmin watches, you have to help them out if you want consistent length counts
Push hard off the wall at the start of a length
Don’t stop, change stroke or wave your hand around during a length
Don’t wave your hand around if having a quick rest between lengths
However, Garmin don’t use their calculated CSS value for anything as far as I know (I wanted to check mine this morning and it isn’t anywhere in Connect website or mobile app that I could find). So you could ignore the length counts and use the 400/200 times to hand calculate your CSS - subtract the 200m time from the 400m time and divide what’s left by 2