The Per Unit Shipping Module works like this ...
If the setting for: Shipping Cost per Unit
is set to 1 then what ever you put in the Product Weight field is going to be:
1 * products_weight
So if you put in Shipping Cost per Unit as .75 and products_weight is 10 then the Shipping Cost is: $7.50 ...
But if you use Shipping Cost per Unit as 1 and products_weight is 10 then the Shipping Cost is: $10.00 ...
This is not set up so that when buying 2 of Product 34 then the 1st is $7.99 and the 2nd one is $6.99 ...
This is not setup that 1lbs = $8.00 and 2lbs = $9.99
You may wish to see if you can do something with tables if that is what you are trying to accomplish ...