All ball pythons have their own growth rate. There is no magic table that says how big your BP should be after a certain number of months (unfortunately).
I have a girl who is 5 months old and almost 300 grams and another girl who is 6 months old and not even 200 grams yet.
I would get a scale, its very helpful to determine how much they are growing. I have discovered that getting their length is pretty hard and really inacurate.