Fix Overall Planned Effort in Hours Planning Duration

Hi,
can I fix the overall effort for a planning duration in tempo planner, so that it is automatically decreased per day if I change the length of the planning period. For example I define a start and end date to have 2 days and I want the effort to be 10 hours in total.
Now when I change the duration from 2 to 4 days the overall number of hours in automatically increased from 10 to 20, but I would like the overall effort of 10 hours to be fixed and therefore the effort per day should be automatically decreased per day, if the time period is extended from 2 to 4 days. Thus, overall it would be still 10 hours of effort.
Thanks.
Cheers,
Steve