I searched for hours for help on 'Default Values for Real Time Shipping Rates" and I'm still confused...

I've done some test checkouts and they all calculate shipping correctly. However, my first genuine Google sale happened today and it came through with the wrong shipping method, and under-charged the customer for shipping.

My question : When using the Shipping Method Generator do I have to specify all my weight bands and how much each weight costs?

Or do I just give it my two shipping methods (Table and Airmail), and let it just calculate one weight band for each ? (seems to be 109g for some reason).