Ok after reading the internet a little I think I understand it more.
VTF set at 0 = Maximum spring force
VTF set at 3.5 = no spring force.
So I don't really understand why the Tecnoweight manual says set at 3.5 and then balance your cart. For me that is impossible.
I'm using the heaviest weight, which is suitable for 6 - 13 grams. My cart is only 8g, yet I can't balance and I'm no where near balancing the cart. I have even shifted the tecnoweight back 8mm on the slide bush like it says in the manual (for carts heavier than 13g!) and I still can't balance.
If I set the VTF dial to 3.5 and leave the weight 8mm back I can get my cart to weigh VTF 1.75g, but the technoweight is far back on the slide bush and the slide bush is really way back on the stub. This seems to contradict the whole point of the TW i.e. close to the pivot bearings.
So I'm at a loss as the best way to set it up now.
Also, my RB700 I have (not set up) behaves the same as my RB301. I have put some little weight on the head and it exhibits the same behaviour.
So it looks like my RB301 is acting normally and 3.5 on the dial is disabling the spring.
Scratch head time