Unless poly, or seriously insulated wood, there will be no bees anywhere near the outsides of end frames (unless getting short of stores) yet. There will only be nine spaces for ten frames, as well. This is bucket chemistry, not analytical!
If I could only weigh 30g with any accuracy, I would simply split the resultant pile into half, or even quarters. It needs to be no more accurate than best you can do by eye.
Or dissolve in water and use half that aqueous solution made (for 15g oxalic), and discard, or store, the remainder.
Look, they say 5ml per seam. A 'seam' in a Langstroth is going to be different than a 'seam' in a National. Clusters are different sizes. So many variables that an average is just that - close enough. There is obviously some lattitude on safe dose. In simple terms that will be more easily exceeded if the oxalic is already very much towards the higher end of the safe range. As Hivemaker has said, probably more risk to the colony if they are nosemic, as long as the guidelines are followed.
RAB