I currently have rules in place to apply a 10% discount if the order total is above a certain threshold, I now need to deal with already discounted items. By that I mean if a product has already been discounted then do not include this when calculating if a 10% discount should be applied. This is what I am trying to achieve in rules:-

When triggered to calculate sell price off a product:-

Add variable to track a total (running total)
Loop through the line items and check state of the discount field (custom boolean field)
If discount is false then add line item amount to variable (running total)
If discount is true then do nothing

Then loop through line items again
If running total is above threshold and item is not discounted then apply 10% discount
If not then do nothing

I am sure this is possible but I can not work our what combination of components to use. Anybody got any ideas?