Here is the screenshot and formulas for each line below, frankly it should be possible to simplify but I could not see how...

Date(2016,6,15)
Date(2016,6,27)
DateDiff(DateValue(Date1.Text, "it" ), DateValue(Date2.Text, "it" ))
Mod(CalDays.Text,7)
Mod(DateDiff(Date(2000,1,3), DateValue(Date1.Text, "it" )),7)+1
Weekday1.Text+CalDaysMod7.Text
If(CalDaysMod7.Text="0",0, If(Value(Weekday2x.Text) < 7, CalDaysMod7.Text, If(Weekday2x.Text="7", Max(CalDaysMod7.Text-1,0), CalDaysMod7.Text-Min(2,8-Weekday1.Text))))
RoundDown(CalDays/7,0)*5+ExtraDays
Basically it is whole weeks*5 plus an adjustment for the extra days depending on how many week-end days they include