By having a calendar template that do not have 8 hour workday, let's say 7,5 hours instead like in Norway, makes the calculation of a project tasks duration and effort wrong, or with decimals (and in my experience the decimals calculation is still being calculated wrong)

I have been told that the design of Project Operations is based on a 8 hour work day, and that this has nothing to do with the calendar template you decide in the project Summery, General section. The connection between the project calendar template and the task board is that the duration may change if e.g. a project resource is away on holiday within the duration time. The duration time will then extend to meet the effort requirement.

My suggestion is that the project tasks duration and effort calculates themself based on the general workhours that is set in the calendar template you choose for the project. In that way a tasks duration and effort may vary in regards to where the project is being executed, like in Norway vs. Sweden.

This will make the system more flexible to use for those businesses that work in different countries. And more important will make the numbers of duration and effort correct when invoicing the customer, giving forecasts and estimates.