Start a 400 m swim, rest 2 min. start a 200 m swim
Use pool length setting 25/50 m and calculate CCS (time pr. 100m)
the CSS calculation, it is:
CSS (m/sec) = (400 - 200) / (T400 - T200)
Where T400 and T200 are your 400 and 200m times in seconds.
then convert your speed from m/sec into time per 100m.
regards
Peter