How do I macro 20,000+ series of data?

[Database Administrator]

The full s/s stretches to nearly 350,000 rows and consists of over
20,000 series of data, between 6 and 30 rows deep, in columns A:J.

In column I of each series, on rows 3, 4 and 5 there are three
numbers.
The second number is to be subtracted from the first with the answer
being divided by the third.
If possible, the answers should appear in the rows directly beneath
these three numbers.

Because there are so many series of data it isn't feasible to manually
input the formula and I'm searching for a macro(s) which can do it
automatically.

Can anyone please suggest code to help me to this end??
Big thanks.
colwyn.