Quote From Kahn:
Hello. If your mark is based on a general average, as was mine, then you calculate like this: 8 modules are worth 15 credits and the dissertation is worth 60 credits. The total credits are 180. However, you can simply think of the dissertation as being 4 15 credit modules. Therefore, add the marks of your 8 modules, then add your dissertation mark (multiplied by four) and divide that sum by 12. This should be your mark.

I made an excel spreadsheet last year during my MSc to calculate final mark and how much I would need in my dissertation but I don't know how to attach it here. You just have to enter your module marks and it tells you how much you need in the dissertation as well as calculating your final mark.