When I cleaned my C9.25 corrector plate I used a mix of Windex and distilled water. It worked very well, no scratches and no residue left on the inside of the corrector plate. There was no special ratio I just mixed a little of the Windex into the distilled water. Some people would probably disagree but this is that kind of hobby.
I have heard people use dishwashing liquid also and they have had no issues.
At the end of the day, making sure you clean away the residue, do not scratch it by using a soft fibre cloth and assemble in the same orientation are the main things.
It is not a hard task to accomplish just take your time and mark everything during disassembly.
Clear skies