The usual approach for preventing duplicates in auto-incremented numbers, which is what you're really doing here, is to generate/assign the number in the Form_BeforeUpdate event. This is the last event to fire before a Record is saved, and if the Record is dumped, will not fire at all, and hence the number will not be assigned and therefore will not be 'missing' from the dumpcustid table.
Actually, doing this task in the Form_BeforeUpdate event makes using the dumpcustid table unnecessary. In over a decade of writing Access apps, using this technique, I've never had a problem with duplictate numbers being generated, and the same approach will prevent the 'orphan' numbers from being entered in the dumpcustid table, as well.
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The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007