Need to divide a value in a column by a set of
running sum values in an adjacent column.
Data looks like this:

Product Month Bal RemBal Result
Calculation:
100 01 $10.00 $10 100%
10/10
100 02 $5.00 $15 25%
5/(10+15)
100 03 $2.00 $12 32%
12/(10+15+12)
......
100 60 $9.00 $9 .01%
9/(10+15+12+ 60 months of balances )
400 01 $8.00 $8 100%
8/8
400 02 $5.00 $35 19%
5/(8+35)

now we have a new value and the month's sequence starts at 1 (01).

Currently the formula is:
row 8 =P8/SUM(R$7:R8)
row 9 =P9/SUM(R$7:R9)
row 10 =P10/SUM(R$7:R10)
row 11 =P11/SUM(R$7:R11)
in this case, the new product started at row 7 and might continue
for 60 more rows, or just 6 more, where the $R$7 part of the formula would
change again.

Usually, every 60 or so lines, (the # of months is different, depending
on the product) l have to re-start the formula (replacing $R$7 with $R$67
for example) so the SUM starts with month
0 and continues summing the contents of column R ,
until the last month for that product.

I have over 24,000 lines of this, it is getting very tedious to have to
re-copy the
formula every time there is a new product., making sure my formula captures
a running sum starting with the first month and divides the value in the
current row
into that running sum.
I'm planning on starting with "IF(P8<p7" - ie a new product has started
so start the forumula using the current row as the cell placed in the
formula
"sum($R$7", start the running sum up,
and divide the contents of the current row into the running sum values until
a new product, where the process starts all over again.
Is it possible to "Anchor" a running sum formula based on changing criteria
with some kind of vlookup/Sumif (or something else)?