ITs all about the middle mark. The problem is for IPT that is put low in comparison to Ext 2 which has it a lot higher. So around that middle mark is a little cluster of marks either side and as the marks get further away from the middle the gap between that and the next mark gets bigger.
Pretty much in reference to scaling think for them picking up the middle mark and moving that alongthe scale higher or lower. (almost always higher).
so if 83 is 90 that we will say that 100 is 100 then maybe 97-99 is 99, 95-96 is 98, 94-95 is 97 93-94 is 96, 91-92 is 95, 89-90 is 94, 87-88 is 93, 86-87 is 92, 84-85 is 91 and 83-84 is 90. Of course we dont actually know if this is true but that would probably be my best guess (or maybe a few 3 mark gaps). Next u might have 1 mark correlates to 1 mark so 82 is 89 and as it gets closer to the middle (usually around 75 i think) you might even get 2 to 1 the other way.
Sorry about the blabble it wa s a poor attempt to explain it.