Unable to Format a Formula number & Display the formula as a column

OK, I've found a first solution for my result format : I'm using the round argument : round ((something), 2), to round the number to the nearest integer with 2 digits :

Hence the code is :

round(avg((select Hand_ins where Class_Group = p.Class_Group).final_grade), 2)

Remains my second difficulty : I wanted this average for all Assignements to appear in the Roster_course table, this is a table where I connect each Student to its classes. I thought that was the appropriate table where the Average of the Student for all the Assignements he passed in this Class should appear. But when I click on "display columns", I can select the "Final Grade" formula, but Ninox does not display it.

Still no solution... and no explaination !

I've created a new Table, just to insert a View Table with the relevant information regarding the students of my Maths Class (Id =  7)

Formula is as such :

select roster_class where Cours = 7

I get a Table with the ID of the roster class (which is basically a link between a Student ID and a Class ID) and the final grade.

Great... but I want to add the Student's name, to be able to export and print the results for this Class. I go in and modify the displayed columns... as soon as I add another columnd, the final grade formula column disappears. And once it's gone, there's no way I can display it any more by modifying the displayed columns... I'm really puzzled !