Start workday,Gantt chart

Hello,

I am trying to create a fully flexible planning time-table for monthly tasks being performed ('planning' sheet, row 40:82).
All tasks are due on a specific working day (column H) and based on the month/year selection the effective deadline dates are calculated in E4:F38 in that sheet.

Everything is working nicely so far, however I'm having a bit trouble on the fine-tuning part:
In order for the project timeline chart (Gantt-chart) to properly visualize on the 'Gantt' sheet I need start dates and duration. Duration is a manual input and the start date (column C) is currently calculated =[@Deadline]-[@Duration]. However this simple calculation would include weekend/holidays, which results in an incorrect chart result.

Just to illustrate using row 43:
The task 'BPC open for input' is due on Monday 03-Feb-2014, 00:00. The duration is 2 work days. The current calculation for the start date in C43 is Saturday, 01-Feb-2014 00:00. However the correct start date should be 30-Jan-2014 00:00 as weekend shouldn't be counted as workdays. So I'd like to visualize on the chart that the task lasts 2 workdays, however lasts for 4 (2 days work, 2 days weekend) days in total.
Additionally I wonder if for 'cosmetic' purposes somehow weekend or holidays could be included in the plot area (currently I just used manual text boxes for illustration).

I hope I could comprehensibly explain the issue and feel free to use what I built so far for your own purpose. I think it's one of my more fancy developments as it'll save quite a bit of time of planning monthly recurring tasks.

Brgrds,
Daniel

Update:
I have experimented a bit around and have now based the chart on a 'correct' start date and end date (latest draft attached). The duration is calculated based on days (additional column inserted). So the chart visualization is better but not perfect yet, as the task length does not reflect the work day time being used.
So I couldn't yet really answer my initial question. Preferably I would still like to highlight somehow the weekend/holiday, but I am also open for other solutions.