Due to a lack of PY data for cats. Original UK conversions used the original classic tornado rating (1.00), took its PY rating (691 I think at the time) and used the SCHRS difference to give a PY to use for the cats. Not ideal I know but some clubs were happy using it. Now I think they use the F18 as the datum point for calculation purposes. You can calculate the same using your US portsmouth rating for the F18 as datum.
Cheshirecatman
Over here, we used the PY numbers from the 2 largest classes, the Hurricane 5.9 (691) and the Dart 18 (798) to get a good balance when we needed to convert other classes from SCHRS to PY.
