Dynamic Rank Measure for Table visualization

I've read a lot of postings on this, but I just can't seem to get it to work.  I have a base table that looks like this:

I want to put this into a visual to and add a measure showing the rank like this:

So when I filter it by A and E for instance, I get:

I've tried this, but I get an error saying it can't find a single value for RankingNumber

RANKX(ALLSELECTED(MyTable),MyTable[RankingNumber],,DESC)

And I've tried these, but everything has the same value for every row corresponding to the count of total rows in MyTable

RANKX(ALLSELECTED(MyTable),MyTable[RankingNumber],MAX(MyTable[RankingNumber]),DESC)

RANKX(ALLSELECTED(MyTable),MyTable[RankingNumber],CALCULATE(SUM(MyTable[RankingNumber])),DESC)

Any help would be greatly appreciated!