Sumif/s maybe! + percentage

This what you are going for ?

Get the sum of anyone that has one of the 4 you listed (21) divided by the count of total found in B4

=(COUNTIFS(Table1[Mark],"1a",Table1[End of Year 1 Target],"<>"&"")+COUNTIFS(Table1[Mark],"2a",Table1[End of Year 1 Target],"<>"&"")+COUNTIFS(Table1[Mark],"2b",Table1[End of Year 1 Target],"<>"&"")+COUNTIFS(Table1[Mark],"2c",Table1[End of Year 1 Target],"<>"&""))/B4

And then the inverse is just 1 - the above or even subtract the end result as it would be the inverse

1-B5

EDIT - The format of the cells may need to change to Percentage instead of the custom format that you currently have set in B5 and B6 however that may be due to region settings in which case it may work as is for you.