Incidentally, rathing than poking around manually setting SPN's just allow the service account to do it automatically.

In ADSIEDIT find you service and account and select properties then go to the permissions tab. Scroll down and find SELF "read\write personal information" and click edit. Scroll down and check the following

Allow read ServicePrincipalName

Allow write ServicePrincipalName

The service account will now automatically register and maintain it's SPN's.