the slice amount will not be used on PaySheet who don't have any model in the YearToDateSlice script.
This case must not appear very often. Just on the old PaySheet (before or during the Upgrade). During the upgrade, a default model is set on the Pay Sheets.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@18613 20353a03-c40f-0410-a6d1-a30d3c3de9de