validation in VF01

Dear All,

I want a restriction in VF01 transaction via user exit or badi where system will not allow to generate bill no if maximum line of FI is reached.

I tried to write this in RV60AFZZ where I checked total line count of XVBRP and if it is above 99 lines system is showing error message.

but for few invoices where line items are 80-90, for those invoices also not allowing to post accounting documents.

waiting feedback from yours if I can restrict users on above conditions.

regards

SP Shukla