On a SA for a self employed, I understand total income on which tax is due is profit from self employment minus personal allowance (£8105 for 2012/13).
Let's say the resultant figure is £15000 (£23105-£8105).
When it comes to Class 4 NICs, do I go £15000 x 9% to get the figure of £1350 for Class 4 NICs OR is it £15000 - £7605 (class 4 lower profits limit) = £7395 then apply the 9% to get £665.55?
OR, is it £23105-£7605 (instead of £8105) to get £15500 then apply 9% to get £1395.
I'm getting confused by the personal allowance and the lower profits limit figures, where to apply what and when. Can somebody please help me.