Clipping back toenails on indoor cat

Question
My cat is declawed on the front claws, but his back toenails are quite long.  I think they should be trimmed, but I am afraid to cut too deep.  I have trimmed the ends with a toenail clipper and actually had a whole shell of the nail come off leaving a new nail underneath.

How can I tell how far I can trim without hurting the cat.

Thanks for any help you can give me.

Answer
Hello Judy,
Thank you for your inquiry. You are right in that the nails need to be trimmed back on a regular basis.  The best clippers are the scissor type specifically designed for cat claws.   The 'quick', or blood supply to the nail, is easily seen in the large majortity of cat nails, as they are clear or white making it relatively easy to stay about 1/16th of an inch in front of it, just removing the curved points.  Cats nails will routinely shed, and when you had the nail 'shell' come off, that was just a matter of timing.  You clipped it in the middle of a natural shed.  That probably won't happen very often, especially when you keep them trimmed back on a regular basis.
Hope this has been helpful.
Tomay