The late DME addition will also boost your hop utilization. It's a good move on your part (you will get a lighter color *and* can probably save some hops), but you will need to run this through some software. Beer Smith can handle the late malt addition.
With hops at $6/oz and Beer Smith at $20 to register, you'll cover the cost of software quickly.
Natural 20 Brewery
Yes, that *is* beer. Water, malt, hops, and yeast mean it's beer. Go ahead and try a glass...