You must start by creating two salary items:

The first one must be typed "Working days and not subject to WT". Nothing else to check.

The second one is typed "WT deduction of the amount submitted abroad".  As for the tab "Applied to", you have to select what should be taken into account. The base salary, of course, but also the other recurring elements specifically used by your company. If you don't want the amount to interfere with the understanding of the settlement, it may be a good idea to give it an item number in the 9000.

For the calculation, enter the number of days concerned: 

Warning: this notion is exclusively daily. So if you have people paid by the hour, it is absolutely necessary that the notion of "Average days" in the management of the employees is correctly filled in. Example: someone paid by the hour normally works 8 hours a day:

FYI: Days are calculated to the 20th according to the guidelines.