Perform calculation with 1st and last number in a column, then 2nd and 2nd last num. etc

[Pat T]

Hi all,
I've only just found this forum. This is my first post.

I am looking for some code to perform a simple calculation using the 1st and last number in a column (ie. subtract the last number from the first number and divide the result by 2)
then perform the same calculation using the 2nd and 2nd last number, then the 3rd and 3rd last number etc.With the result of the calculation being output to the cell adjacent to the first number used in the calculation.
The column of input data will be of varying lengths and the numbers in the column will be separated by blank cells of varying intervals. there will always be an even number of entries.
Please see attached example to better explain my problem.

Thanks.

[jindon]

Try

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[bebo021999]

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[davsth]

as a formula it is a bit ugly

in b2 as an array so enter with control enter
=IF(OR(A2>="",COUNT($b$1:b1)>=INT(COUNT($A$1:$A$43)/2)),"",(A2-INDEX($A$2:$A$43,LARGE(IF($A$2:$A$43<>"",ROW($A$2:$A$43)-1,""),1+COUNT($b$1:b1))))/2)

[Pat T]

Thanks jindon. have tested and works great. you don't know how long i've been trying to figure that out. so thank you.
awesome.

[Pat T]

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Thanks for the solution bebo021999. This does the job.

[Pat T]

as a formula it is a bit ugly

in b2 as an array so enter with control enter
=IF(OR(A2>="",COUNT($b$1:b1)>=INT(COUNT($A$1:$A$43)/2)),"",(A2-INDEX($A$2:$A$43,LARGE(IF($A$2:$A$43<>"",ROW($A$2:$A$43)-1,""),1+COUNT($b$1:b1))))/2)

Thanks for the suggestion davsth. its nice to have a formula option too. so thank you.