Beast Mode to Fix Revenue per Item at the first entry (so compare the rest)?

Imagine the following:

Item Revenue (Vs 1st Entry)

A 100 0

B 80 -20

C 60 -40

Any ideas on how to write a beast mode that first fixes the first entry of the rows to a constant value for all rows (then writing a companion beast mode of the variance row by row is trivial)? I've tried to SUM(SUM(Rev)) OVER (PARTITION BY Item) but it doesnt seem to work.