Not sure that is the right calculation.
Initial statistic T were trans
Of which 0.29T were mfl and 0.71T were ne.
0.71T should be 'correct' as true mumber of ne speakers
But actually ahould be 0.9 of Correct trans.
so 0.71T = 0.9C
T = 0.9C/0.71
so 0.29T is actually 0.29x0.9xC/0.71=0.367C
So number over calculated X is
0.29T-0.1C which is 0.367C-0.1C = 0.267C
X = 0.267C = 0.71/0.9 T X 0.267=0.21T
Ie overcalculation is
X=0.21T
So I think you can only discount around .2 not nearer to 0.3.
NB
- I have only done this because FWR prides itself on accuracy, not to nitpick.
- This is assuming that non native English speakers have no reason to be less trans than anyone else, which of course may not be true.
- Someone else can check my maths, it is early in the morning!