Sum( NetWorkDays()) gives unexpected result

[#1]

sum( {$<[ResourceId Hours per week] = {">0"} >} (NetWorkDays($(vBeginDate4), $(vEndDate4))  * 8 * 60 * $(vTimeUnitFactor) * [ResourceId Hours per week]/40) -
if([Unavailable Start Date] < $(vEndDate4) and [Unavailable End Date] > $(vBeginDate4),
      (NetWorkDays(if([Unavailable Start Date] < $(vBeginDate4), $(vBeginDate4), [Unavailable Start Date]),
           if([Unavailable End Date] < $(vEndDate4), [Unavailable End Date], $(vEndDate4))) * 8 * 60 * $(vTimeUnitFactor) * [ResourceId Hours per week]/40 )
      , 0
      )
)

What i,m  trying to do is calculate nettoworkdays: totaal workdays - unavailable days

The unavailable days are in a separate table and link via a ResourceId?

The problem is, when there are only 0 or 1 unavailable days records in a specific period, the calculation works fine.

But when there are more I get enexpected results e:g

Numer of  unavailable days = 18

Bruto workdays = 92

Expected = 74

Count( 'above expresion' ) gives = 3

So I assume that the problem somehow is related to the number of  unavailable days records.

Gerry