I have a range of numerical data in a column sorted. I ned to rank these for
an "if" arguement.

I need to know the top 5% rated (A), next 10% (B), next 60% (C) then the
last 25% (D).

The data volumes are varied between 5 rows and 21 rows and the values
normally are anywhere between 50 and 200.

I have tried various approaches but am probably missing an obvious one...

Somewhere, put a 4 line 2 column database. Say from AA1:AB4.

First column consists of:
0
=PERCENTILE(E2:E25,0.25)
=PERCENTILE(E2:E25,0.85)
=PERCENTILE(E2:E25,0.95)

2nd column:
D
C
B
A

Back in your original database make a new column with a vlookup, copy
to each cell:
=VLOOKUP(E2,$AA$1:$AB$4,2)

Although this is only one way to decide on the ranking. I'm not
exactly sure how you rank your numbers. May have to alter that
percentile formula to something else.