Conditional format: compare 2 columns with grades [a,b,c], change colour based on >,<, =

I'd like to compare the contents of column K (student result) with column A (student target), grade set: C, D, E, F, G, U.

If column K=A, fill cell colour yellow
If column K<A, fill cell colour red
If column K>A, fill cell colour green
If column K=null/empty, no colour fill.

I can manage this fine comparing one cell with another but when I attempt to change the range over 100 cells, it doesn't work as expected. Any help much appreciated, I'm guessing its $ absolute cell ref brain block?

Please find attached.