How much weight loss to drop dress size?

When I started on 3/7 I was 16/18 bottoms and definately a 18 on top.

I have now lost 15lbs and 2-3 inches all over. I am now wearing 16 tops and 14/16 bottoms.

Just wondered if there was any rule as to how much weight drops dress sizes?

I found it about a stone a size...give and take a few pounds. The last couple of stone didn't make much of a difference to my size, just 'refined' me a bit :D
I went from a size 16 and having lost 2 stone went to a comfortable size 14. I just hope that I break into the 12's when I get down to 11 stone something?
I'm finding it about 1 size per stone lost having gone from 17st 10 to 13st 1.5lbs.

Given that I am in a 16 at the moment and in another 3 stone will be where I was when I was a size 10 then this should hold for the last half of the weight to be lost as much as the first :)