A quick, simple formula does not really work when many rate bands are involved in the calculation.
Examples of how the percentage of your gross to net can change with increasing earnings;
Gross < �5200 ... Net 100%
Gross � 6850 ... Net ~95%
Gross � 8650 ... Net ~90%
Gross �11050 ... Net ~85%
Gross �15300 ... Net ~80%
Gross �24850 ... Net ~75%
Gross �51550 ... Net ~70%
Gross �94500 ... Net ~65%
Accurate answers are obtainable using online calculators such as the one above or this...
http://www.listentotaxman.com/index.php?year=2 007