Here's what's known about the new grading:
Roughly the same proportion of students will get a 1 or above as got a G or above last year.
Roughly the same proportion of students will get a 4 or above as got a C or above last year.
Roughly the same proportion of students will get a 7 or above as got an A or above last year.
Percentage of those achieving at least grade 7 who should be awarded grade 9 = 7% + 0.5 × (percentage of candidates awarded grade 7 or above).
The proportion of students sitting higher and foundation is unknown, and the proportion of the grades 4 and 5 which will be awarded on each paper is unknown.
Once the boundaries for 1, 4, 7 and 9 have been set, the other grades will be set between them at equal numeric intervals regardless of how many pupils each interval will contain.
Now go away and predict grades for your pupils without any knowledge of how the rest of the country is doing.