MS PRO How to assign 20% of total quantity of all related resources (material, work column) to selected task assignments?

Is it possible? By macro any other technique?

Example 

Task_1 is selected task. user input is 20%

Res_1 (Work 100,material) and Res_2 (Work 80, material) are selected resources

I would like to create/update task resource assignment to have result:

* Res_1XTask_1 - 20 units

* Res_2XTask_1 - 16 units

Best regards

MarN