Map values from Child Entity to Parent Entity real time

Hi everyone!

I have this request.

I have created a Package entity which has the fields Number of Visits. On my Case entity i have a lookup field on the Package Entity and a field Remaining Number of Visits.

What i want is when i choose the Package from the lookup , to map the value of Number of Visits to my Remaining Number of Visits field on my case. Although i have done this with a workflow , it won't show the value real time like Javascript. It requires a number of refreshes to bring that value.

Is there a way to do this and display the value instantly ?

Thanks in advance!