Help I'm totally baffled by this formula.

I am working on a holiday database for our staff and need to stop employees
booking holidays if they dont have enough hours.
I have a main form which holds their entitlement, totals from 3 sub forms
(holidays booked, holiday adjustments and bank holidays worked) in order to
calculate their remaining hours.

I want to put a control onto the sub form where they book their holidays to
stop entitlements going below 0.

I have based my sub form on a query that will pick up the total holidays
booked and the original entitlement, but need a way to pick up total holiday
adjustments and total bank holidays worked.  I have a feeling that the dsum
function would work, but dont understand the structure of how i put this
together.  I have had several attemps and seem to be tying myself in knots.

Would be really grateful if someone out there could help.