since you already have a column that is storing the shipping rate, this is fairly strait forward:
on the calculation tab, create a totalShipping calcualtion similar to the totalPrice calcualtion, but use the shippping column:
[ShippingRate] * [Quantity]
this example uses a column named ShippingRate, if you use a different name, adjust the code accordingly
Then create the shipping rule as:
triggers:
"Subtotal for any column"
if the subtotal for column "totalShipping" is ">" a value of "0"
AND
"Subtotal for any column"
if the subtotal for column "totalShipping" is "<" a value of "50"
calculation:
"Based on Multiple of Column subtotal"
subtotal of column "totalShipping" times "1" "plus" "0"