Calculated field or LOD

Hi All,

I wonder if someone can help. I've not long started with Tableau and I am learning to do some amazing stuff that will help me when I start my degree in September. So as you'll appreciate I'm still at the bottom odf a steep learning curve. I'm having a problem resolving an issue with data I have been working in Tableau. I've attached the worksheet and also a spreadsheet that has the maths worked out but shows a different %. I realise therefore it must need me to write a calculated field or LOD. An thats where I break down. I wonder if someone could have a look and see what they think and maybe give me some ideas. Many thanks.

Sean