How to predict new disk allocation?
Mongo always allocates next file when perform a first write to last file.
So when collection is created it has 64MB allocated (using small files option)  in first file and mongo allocates 128MB in next chunk.

When collection grows and last file has 2GB, then when document is added to that file, next 2GB file is allocated.

file allocation sizes
e.g. 64MB (small files), 128MB, 256MB, 512MB ... 2GB - this is max size and every new one will be 2GB.)