Calculating Running-percentage or percentage growth

[someonear]

All

this is the  scenario I want to create

Week - Amount  - Growth % 
1       - 50        - NULL
2       - 40        - (40-50)/50
3       - 80        - (80-40)/40
4       - 90        - (90-80)/80

I am using a list report in 8.2 environment (Report Studio)

I have created a separate col. X  where X = running-difference([amount] for [week])

and now trying to create Growth % as  =  X/(Amount-X)

Though X is displaying the correct value but the numbers for Growth %  looks to be off

Is there any other way  to solve this issue?

Thanks much