Formula to quantify percent occurrence of maximum value per year in each month

Hi. No-one is going to want to retype your data; so please attach a spreadsheet, instead of a picture of one...

It is - (at least for me) to test solutions. Actually I got bored and mocked up some numbers... I couldn't think of a one-step solution. Is this approach OK? If so, I'll put the formulae into your workbook. If not, I'll have another think...

Using your own data, assuming a list of years, commencing in G2, this array formula returns the month of maximum discharge each year:

=INDEX($B$2:$B$325,MATCH(MAX(IF($C$2:$C$325=I2,$D$2:$D$325)),$D$2:$D$325,0))

This then returns the %s, assuming the list of months begins in L2:
=COUNTIF($J$2:$J$94,L2)/COUNTA($J$2:$J$194)

See sheet.

I forgot to add...

Array Formulae are a little different from ordinary formulas in that they MUST be confirmed by pressing CTRL+SHIFT+ENTER to activate the array, not just ENTER.

You will know the array is active when you see curly braces { } appear around the outside of your formula. If you do not CTRL+SHIFT+ENTER you will (almost always) get an error message or an incorrect answer. Press F2 on that cell and try again.

Don't type the curly braces yourself - it won't work...