Can you please check the rounding in Data>Manage Fields for this field.
First total should be 8.2 , (not 2.6).
Other total 10.4 is correct.

First Total Should be 816/308= 2.6 second total should be 478/288=1.6
The subtotals should not aggregate values.

Could you please send a file or a sample? Will need to look at your formulas. Thanks

What happens is that I have this calculated field (Pages per Visit) defined in one of the Field Organizer blocks in Data Manager. I need the field in my grouped Table View. The table has aggregated values so I had to specify the way of aggregation for field. The aggregation for this field is weighted average by Visits because there is no option to apply formula defined in the Data Manager.
If there was such option it would solve the problem.

When you define formulas in the DataManager, results are imported as static values into Omniscope. If you want these formula fields to respond to various aggregation options, it might help if you move them 'downstream', and define formula fields in Data Explorer.
Without seeing the file, I would have to continue to demonstrate what happens when you don't see the whole picture...

When I change the field into a calculated field and apply formula then Subtotals show the correct value. The problem is that apart from the calculated field I have to add to the table all fields used by formula (which makes table quiet big and these formula components-fields don't give any insight to the report).
Is it possible to use formula field without showing its component-fields?

Yes - please use the Field picker in the Table View and hide all unwanted fields (from the view), including those used to calculate the formula values. This will just hide them from that view, without effect on the dependant variables.

When I deselect the unwanted fields this error shows up (see attached)

Clarification: formulas recalculated upon grouped/summary data in the table view require all input fields to be present. Consider using view aggregation instead, which doesn't suffer this limitation, or shrinking columns to near-nothing, or using a Content View with scripting.

Thanks Steve,
I will shrink the columns - the easiest option.