Big Spender I want 20% and its taking 10ish%

[SarahL]

I am paid and set up Big Spender - very simple thank you

for now I only want a simple

Spend over £100, get 20% off

so I aded this code in order_total

         $this->add_threshold(100, 'Spend over £100, get 20% off', false); 
         $this->set_cart_discount("%",20); 

But when I get to the checkout this is what I see:

Sub-Total:£304.00
Shipping:£0.00
Discount :-£34.40 <- big spender here
Total:£269.60

As you can see that is not 20%?

The set up in order total modules is:

This module is installed true
Sort Order 294
Include Tax false
Re-Calculate Tax Standard

Any ideas - a new install of 1.3.8a used

thank you
Sarah