Italy Tax Exemption amount reached warning notification

We have tax exemption certificates for Italian customers in Receivables, data held in the Customer Tax Profile -> Tax Exemptions. We need to get a warning email for the Italy Tax exemptions, but instead of dates, we need it to be when the account has been billed close to the limit of the exemption.

E.g. Fratelli Rosetti (50277) has a limit of 200,000 Euros. We need to alert when they have been billed close to this, i would suggest around 50,000 before the limit.