calculating file intervals from 300 to 300

Hello experts!

Here is a difficult one!

I have a column O for inserting the number of document files that is registered in my workbook (ex: 3 files).
In column P a have a formula that calculates the interval between files (ex: from 1 to 3), which is:

=IF(OR(Q2>Q1;Q1="Folder's number");"1-"&O2;SUBTOTAL(9;O$2:INDIRECT("M"&ROW()-1))+1&"-"&SUBTOTAL(9;O$2:O2))

In column Q I have a formula that calculates Folder's successively number if the files rich the maximum of 300 (ex: volum 1 for the first interval from 1 to 300, volum 2 for the next interval of 1 to 300, and so on):

=IF(P1="1-"&O1;Q1;IF(SUBTOTAL(9;O$2:O2)<=300;1;Q1+1))

The problem is that the formulas work only for the first interval. When the files gets to first 300 and is needed to recycle the hole process for the next interval of 300 files, which should be in volum 2, and so on.

I've attached the workbook.

Any ideas how can be solved?
Is there a formula for this, or is necessary a VBA code?

Thank you so much!