I have a grid that contains the columns: Quantity (numeric), From(date), To(date), Total(custom format)

I would like to insert into the column Total the result of doing: Quantity * datediff(From, To)

The result needs to be formatted in the following format: HHH:MM. Example 120:30.

My questions are:

1) Is there any formula, or group of formulas that I can use to achieve this goal?

2) How can I configure the Grid to recalculate total automatically (on client side) when Quantity is changed?

3) Is there any Manual or Reference guide where I can take a look to the list of functions that I can use inside Formula Field?

            myGrid.CalcManager = calcmgr;
            myGrid.Columns.FromKey("Total").Formula = "????"; // What should I put here?

Thank you