Auto Number when record is added if certain field is Yes

I'm not quite sure where to start with this one.

I need to have an auto number added, but it has to be in text format to hold leading zeros, when a new record is added when a formula field changes to yes.

Technically I'm creating a dummy tin that I need to start with 000000150
So when a child record is added and the formula field on the parent record changes to yes, then I need to add this dummy TIN on the parent record.  I need it to go to the next dummy TIN on the next record when the same situation happens. 150, 151, 152, etc.

I'm thinking I might need a table that has a bunch of Dummy TINs and a field to be checked of when used but that is just my guess.

Any Ideas?

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Carol Mcconnell
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