Start by modifying the number of hours salary item:

Check the "Calculation of monthly base salary according to applied to" box.

In the "Applied to" tab, check the items you wish to take into account (e.g. base salary, certain benefits in kind, etc.).

It will then take all these elements into account when calculating the hourly rate.

Warning!  This setting is only valid for monthly salaries.

Note: if you leave the item in its standard setting, by default it will only take into account the base salary.