Going On All Year Long Mystery Cache

Going Puzzles are puzzles that require the solver to use logic. Once you have found the solution, you should be able to solve the puzzle simply whilst out in the field. Quite handy should you forget to add the corrected coordinates to your GPS, or should the final location change! 

This puzzle is a pure logic puzzle. No need to go searching the text here for hidden clues, although if it makes you happy, don't let me stop you! No, this is all to do with sequences.

Below is a list which you may recognise as being the months of the year. For six months of the year, I have assigned a number in ascending order. All you have to do is work out what the numbers for the other six months of the year should be. Once you have done that, then just put those figures in the relevant places et voilà, you'll have the final coordinates. 

The cache can be found at N50 5A.BCD W000 2E.FGH