Average the top 80% of the total data

[OtterBA]

I have one column of data (A:A) in b1 I want to have an average of the value in the top 80% of all the data in (A:A). I can have a conditional format highlight the top 80% but I want a formula to sum and count all cells in the top 80%.
Total:15
Average:2
Total*.8 = 12

Example
.5
.5
1
5
5
.5
2
.5
So... 5,5,2 are the largest numbers =12 but the average is 4 I can do this by hand for this little data but I have huge rows of data are not in order. I'm thinking there needs to be an array formula to do this but not sure what it is.

 

[Jonmo1]

It's a little unclear how you've determined 5 5 and 2 as the top 80%

If I use the percentrank function on that sample data set, it tells me only 5 is in the top 80%

Excel Workbook
	A	B	C
1	0.5		0.00%
2	0.5		0.00%
3	1		57.10%
4	5		85.70%
5	5		85.70%
6	0.5		0.00%
7	2		71.40%
8	0.5		0.00%
Sheet1

 

[OtterBA]

5,5,2 add up (12) to 80% of the total of 15. No formula was used I just know that those three are the numbers I want averaged because they are the largest numbers that toal 80% of 15.

So... average of the largest string that = 80% of total. or something like average(Large(A:A),sum(A:A)*.8,A:A) -

 

[OtterBA]

Here is a formula that gets me close. but I need something more dynamic.
=SUM(LARGE(J3:J10,{1,2,3}))/COUNT(LARGE(J3:J10,{1,2,3}))
instead of {1,2,3} i need something more like sum(j3:j10)*.8