I have an excel sheet where I want to be able to select a range of cells in a
column and have excel automatically number them based on the value in a
previous row.

e.g. if A1 = 1, and I select A2 through A4, I want to be able to
automatically number A2 to A4 as 1.1, 1.2, 1.3. The idea is that if I then
insert a new row between A2 and A3, I could select the new A2 through A5 and
renumber as 1.1 through 1.4.

I hope Ive made it clear what I am after.