AutoNotification How to set and use Notification Id

I have a number of profiles now that are successfully dealing with notifications and I'm looking to improve them by setting my own notification Id to allow better control. However, when I set a value on create and then query it I don't get the same value for %anid and the subsequent action (e.g. a update) fails with the message:

Notification can't be updated because no notification with the same ID (24)exists.

Even though a flash shows the value as 24 after the query. If I actually set the value to 1 then the flash shows 49.

What are the rules around setting the Id, what values are allowed?
It's driving me nuts.