"alot of the flying schools will knock off 5 hours off the IR once you have done your CPL"
This is correct but bear in mind that you need to have met the requirements for CPL "issue" which is 200 hours total in order to get the reduced IR.
You don't have to have the CPL issued to do the IR, but you must have met the requirements.
Back to your original question, there is no right or wrong way, but trust everyone who tells you it will cost more than you originally budget. You need to factor in extra training hours, possibly an exam resit, and maybe a partial of a skills test.
Best of luck
Camel Toe
"Over Macho Grande?"