Shipping setup issue - incorrect values

I cannot seem to set up a shipping method that can deal with the following:

0-10kgs - Flat rate R88.39

10kgs - R4.50 per kg

i created a shipping method and added the following weight dependencies:

more than 0kgs - R0.00 absolute

more than 5kgs - R88.39 absolute

more than 10kgs - R4.50 per kg

when I add a 40kg product to the cart, the shipping cost shows R180.00 instead of R88.39 + R135.00 (4.5 x 30kg) = R223.39

Any ideas? Is this not possible or have I made an incorrect configuration?