With a task starting today (Tuesday 30 Sept at 08:00) with 10 days duration using the Standard calendar - the task ends 13 October at 5:00 pm.

Assuming you have made the "7 Days Calendar" also as 08:00 to 17:00 and using your formula - I see 14 days in the custom field.

When I apply the 24 hours calendar, the duration in the standard duration field still reads 10 days. The task begins 30 Sept at 08:00 and ends Oct 3 at 16:00. The duration in the calculated field is 3.88 days.

Regarding task type - as I said it does NOT play into task duration unless you have resources assigned. If you have resources assigned then the "fixed" value is not altered by Project when you alter one of the other values. For example:

Fixed Units task type:
you change Work, Project recalculates Duration
you change Duration, Project recalculates Work

Fixed Duration task type:
you change Work, Project recalculates units
you change Units, Project recalculates Work

But again - this only applies if you have resources assigned. If you have resources assigned and you are using Task Calendars, unless you check the box "scheduling ignores resource calendars" the task will be scheduled during common working time between the task calendar and the resource calendar.

If possible, I suggest getting some hands-on training in Project. It is by no means an intuitive tool. I can only help so much by answering questions you have been posting. A good 2 to 3 days course will help clear up many questions.