I did a brief test on my 7870, 7870xt and 7950.. The formula failed on all those.

I have tried differed TCs a bit, and currently I get 78 kH/s with 4096 TC on 7870xt, and 72 kH/s with 4096 TX on 7870. Both are clocked at 1150/1000 (GPU/Mem). These are the highest / best performing settings. Intensity is 11 on both. With higher intensity I get better hash rate, but there are more HW errors.

On my 7950 I now get 67 kH/s, I haven't been able to get better performance out of it. TC is at 7936, intensity 11.

BTW. I think it would be useful to add a percentage of HW errors of all processed blocks. That way one could see the effective hash rate when taking HW errors into account.