Calculated Total carried out after an append query.

[jfaz]

I have created a database for staff to book holidays.

I have a form on which there is calculated field (holidays remaining) based
on a sub form (holidays booked). The holidays booked table is populated from
an append query - this is to stop staff from changing the record once it has
been input. (this is done one record at a time).

I wish to test the holidays remaining field before anyone can book holidays
taken so that if this booking brings them below zero they are stopped from
booking the holiday.

Any help would be welcome.