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You have picked quite a difficult task for yourself to start out with. Here is the formula that will do what you want.
=INDEX({"A","B","C","D","E"},MATCH($B16,{1000,0.95,0.9,0.85,0.8},-1))
To understand this formula you (after you look up the INDEX and MATCH functions) you need to know that data between curly braces are ranges. You could write the letters from A to E in A1:A5 or A1:E1 and then replace {"A","B","C","D","E"} with $A$1:$A$5 or $A$1:$E$1. In the same way you could replace {1000,0.95,0.9,0.85,0.8} with a range of worksheet cells containing these same values. To do so would be better for a number of reasons the most important of which is that formulas should manipulate data, not contain them. However, for explaining what is happening the above view is much preferrable.
BTW your formula seems to contain a logical error which Excel's MATCH function has difficulty duplicating. You want > 0.95 = "A" and <0.95 to be "B". My above function complies with your first condition but grades =0.95 as "B". If you want >=0.95 to be grade "A" you must reverse the MATCH part of the function and build the INDEX array accordingly.
=INDEX({"E","D","C","B","A"},MATCH($B16,{0,0.8,0.85,0.9,0.95},1))
Observe the final "1" or "-1" in the MATCH functions.