One thing all ratios have in common is that they don't have units.
Inches, millimetres, Nelson's Columns or Titanics all become irrelevant for a ratio.
I wish LR would allow custom ratios to be favourited.. so you could enter a custom ratio as needed whilst retaining your most frequently used ones. I know it could be done with a preset, but this seems like using a sledgehammer to crack a nut.
It should retain the memory of your custom crop sizes under the 'Enter Custom' line on the drop down menu. I'm not certain how many it stores though.