Hi,
This functionality it's already placed in the XT00 time schema in custom PCR because you are saying it's currently calculating as salary/26 .So its already present in time subschema inside the payroll schema,
Check and find out that pcr and inside that PCR might be one operation is used as amt/26, change it to amt/30.
Hope so it will work or else share that pcr and will say where to change.
Regards,
Sankarsan