Calculating CGT on part let, part lived in property

Forward planning scenario:

Property purchased in 1996 for £70,000
Main residence for 3 years to 1999
Let for a total of 8 years from 1999 to 2007
Sold for £220,000 in 2007
Total gain £150,000
Asset to be held for 11 years

Ignoring indexation allowance and on the basis of my understanding that
- the final 3 years are tax free
- there is an additional letting relief allowance of £40k
- non business assets held for over 10 years are taxed at 24%
- tax rules won't change!
I have worked out the likely tax bill as follows:

Gain: £150,000

Taxable gain before allowances: £68,182 [£13,636 (1/11th of gain) x 5 (taxable years)]

Less allowances: £47,200 [£40k plus personal allowance]

Taxable gain: £20,982 [£68,182 - £47,200]

Tax @ 24% = £5,036

I look forward to your comments

Many thanks

Sharon Jordan