Yeah I am sure the chances are not 70% across the board. There are certain people with JS1's who have a 99% chance of a JS2 (based on their numbers) and another portion of people who probably have less than 20% chance (based on their numbers), and obviously some who are on the bubble where the interview could make the difference between an admit/WL/deny.
It would be really interesting to get the data on this, but unfortunately that will probably never happen.
I'm sure it wouldn't be fully representative, but LS22, in a few months (at the end of the cycle) is it possible to break down the data in your spreadsheet to help determine this kind of stuff? Somebody that gets in could post a link to the spreadsheet in the Facebook group for HLS admits to increase the sample size.