The best way to home in on the figure you need is to ask yourself a series of questions.
You shouldn't forget to take practicalities into consideration such as how much can you reasonably afford to pay towards a policy each month. This should be a balance between ensuring the cover you need is in place and leaving enough disposable income for enjoying life - after all the best outcome is a claim never needs to be made on life insurance.
You should be clear on your current policy position before you start to make sure you're not paying out unnecessarily.