Absence Type Validation Formula

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Hi, has anyone used a validation formula in an Absence Type to cap the duration to x number of days in total in the current calendar year. I know we can restrict the max duration per request using the config options in the Absence Type but I need to know how many might already have been used or booked in future before allowing a new request. I assume a FF will do it but if anyone has an similar example I'd be grateful for a steer. 

Thanks in advance,

Linda