Insulation thickness

How about this free program -

http://www.pipeinsulation.org/

It all depends on what you mean, but 8" should see you right in the UK. As long as you're happy with it, that's the main thing.

Providing a tad more info might help ?

As I was taught. If you can put the palm of your hand and stand comfortably - thats 95% good insulation. The thicker the density the higher the temperature of the insulation material you save on material cost getting a smaller sft area with the insulation & cladding.