You seem to be overly concerned with:

ROW(rng)-MIN(ROW(rng))+1)

You could put this portion in a separate cell and then refer to that cell:

=MIN(ROW(rng))+1

You need to understand what this is doing.

When you index a range the values of that range are in specific positions.
The positions are numbered 1 through the total number of cells in the range.
Your range is C5:C7. So:

C5 = position 1
C6 = position 2
C7 = position 3

In order to get the correct result from the formula we need to tell the
INDEX function we want the value located at position n. We do that using
ROW(rng)-MIN(ROW(rng))+1).

We have to convert the actual row numbers of "rng" to correspond to the
position numbers of the indexed range.

The actual row numbers of "rng" are 5,6,7. We need to convert those to
1,2,3. Here's how we do that:

ROW(rng)-MIN(ROW(rng))+1)

ROW(C5)-MIN(ROW(C5))+1 = 1
ROW(C6)-MIN(ROW(C5))+1 = 2
ROW(C7)-MIN(ROW(C5))+1 = 3

The *only* time you could replace ROW(rng)-MIN(ROW(rng))+1) with just
ROW(rng) is *if* the actual indexed range started in row 1. For example, if
the actual indexed range was A1:A3. In this case the actual row numbers
naturally correspond to the position numbers on the indexed range.

A1 = row 1 = position 1
A2 = row 2 = position 2
A3 = row 3 = position 3

However, if the actual range was A1:A3 and you used just ROW(rng) and
inserted a new row 1 the formula could return incorrect results because now
the row numbers do not correspond to the position numbers of the indexed
range.

Using ROW(rng)-MIN(ROW(rng))+1) accounts for this. This is the most
fool-proof method to convert the actual row numbers to the correct position
numbers.

exp101