100% UMS is calculated by doubling the distance between the A boundary and the B boundary and adding it onto the A boundary.
In this case the distance is 8. 8*2=16 Add that onto 64 and you get 80, which in this case if full marks anyway.
Hence, if you get 79/80 on your coursework, that is 12/13 of the way between the A boundary and the full UMS boundary (80/80 in this case). Hence your UMS score will be 12/13 of the way between 64/80 and 80/80. Therefore your score will be 78.8/80.
Just to note, if you calculate 100% UMS to be above full raw marks, you assume that 100% UMS is equal to 100% raw marks.
Check out this page for full details:
http://www.thestudentroom.co.uk/wiki/UMS