Finding The Biggest Value In Custom List?

I know how to make customs lists, but it's not getting me anywhere.

Let's say I have a lot of data containing values as follows and in the following list I sorted them from small to large:

5a
5a+
5b
5b+
5c
5c+
6a
6a+
6b
6b+
6c
6c+
7a
7a+
7b
7b+
7c
7c+
8a

and so on... In the list, the date isn't sorted from small to large, many values (if not all) occur multiple times and on occasion some values are added to the column. With that last part I mean that you can see it as a batch of 500, and if so far the batch only contains 450 values there will be (for the time being until items get added) 50 empty cells in the column as well.

You can't order numerically or alphabetically. I know I can make a custom list and then just sort according to that custom list so the biggest value would come out on top or bottom but that's not what I'm looking for either, as the data needs to be sort by another colomn (dates in chronological order).

So I'm just looking for a formule saying "return the biggest value if the order of things is as I typed above'.