secondary charge cost base on original shipment cost

Hi all,

I need to create secondary charge shipment with 1% of original_shipment cost (rate record for original = 20 eur per shipment).
I have created secondary charge rule.
What kind of rate basis item I must select in rate record for secondary charge to get right result?
I must receive 2 eur (1% from 20 eur)

any ideas?

Thanks