Is there a way of using number from TOT (total) column in a formula (from relationship table)?

(My Hunch)

If your Child table is called 'Delays' I'm going to assume your parent table is something like "Projects".

To get that total value that appears in the embedded delay report, usable in a formula, you will need to make a summary field.

In the table settings, find the relationship between the project and the delays
>
Click "Create Summary"
>
Select "Total" and the field (i.e. Duration)
>
Create and name as you'd like

Then you should be able to use wherever you'd like.