You need 500 hours total experience when you apply for the Class Rating Instructor rating.
There is no getting away from the 500 hour requirement. However, the flying time during the course counts towards the 500.
JAR-FCL 1.330(e) and 1.380(a) are the references.
If you have done a CPL, and have some instructional experience on top of that then you can't be far from the 500 total time mark.
Regards,
DFC