Is this for every item or just the one?
If it is for every item, then you update the Total Price calculation to:
TotalPrice = ([Price] * [Quantity]) - floor([Quantity]/6)
And then maybe add a few calculations so you can display them on the page where you need to:
DiscountAmount = floor([Quantity]/6)
Now if you didn't want the discount to be applied automatically in the row, but want to show it below, then you wouldn't update the TotalPrice calculation and you would just add the DiscountAmount and define a Discount rule for the total of that Column.
I assume you use Quantity for your length in feet... if not substitite it for the eCart column name you are using.