It seems that when I step through the code in 1.7 I can see that the value of the discount is being calculated at the full price of the product and then applied to the tiered price which results in too large a discount.
E.g.
Discount of 15% applied to Customer Role 1, Product SKU (XX)
Product sells for 12.95 if sold in quantity up to 50
Product sells for 10.00 if sold in quantity over 50
The calculations mean that the 15% discount is applied to 12.95 (1.94) and applied to the initial price of 10.00 delivering a larger discount on the already discounted product.
