Linking field in main form with field in subform to perform calc

[Ammo]

Hi All,

I have created a main form called 'school_details_frm', which consists of
the following fields relevant to this problem I am trying to solve:

CostCentre (primary key), SchoolName, VisitHoursPurchased,
PurchasedHoursRemaining (new calculated field added on main form, but not in
table)

The form consists of a subform called 'visit_details_subfrm', which has the
following fields linked by CostCentre field (1:M relationship):

SchoolVisitNumber (primary key), VisitDate, VisitType, VisitStartTime,
VisitFinishTime, HoursCompleted (calculated field added on main form, but not
in table, currently has the following control source formula:
Format(DateDiff("n",[VisitStartTime],[VisitFinishTime])/1440,"hh:nn"))

I would like for the 'PurchasedHoursRemaining' field on the main form to be
calculated as: VisitHoursPurchased - Sum of sub records (HoursCompleted)

One way I tried this was by adding a new field in the subform called
‘VisitStatus’ (yes/no data type) and adding the following control source code
in the 'PurchasedHoursRemaining' field on the main form:

=[VisitHoursPurchased]-DSum("[HoursCompleted]","[visit_details_tbl]![VisitStatus] = True and [CostCentre] = " & visit_details_subfrm.Form!CostCentre)

However all I receive is an Error# message in the field. Can anyone help?
Many thanks.

Kind Regards

Ammo