Check in filled values are correct

Hello,

I have a web interface with two layouts, the first one is just a description and the second one where you enter data.

For the key figure concerned, they have to reach the number showed in the first layout distributing it in the second layout by diff characteristics.

I would like to make a check before the data is saved.

I would like to show a message like "the sum of the values you entered is greater(lower) than the numbers of employees in your team" ... etc...

Maybe a user exit function is to be created, but where to place it ? In the after data change function in the web application builder ? And how to generate the message in the user exit ?

Any help is welcome.

Regards.