Not to be contrary....but more for understanding, be careful with simply "going by the book" for double mating....I do double mate my Columbian Rocks in similar fashion to what you describe for SPRs...but, genetics can be goofy things. I have a pullet from my male pen (2013 hatch) that will DEFINITELY be shown....by far the best female I have produced yet in 4 yrs with this line. While it looks good on paper, every now and again you might get surprised!!
I will be keeping records of each breeding pen so I can keep track of which pens is producing the best offspring. I am looking forward to using the double mating system on my Silver Penciled Plymouth Rocks. I have older books that go into detail on double mating the Silver Penciled Plymouth Rocks and it looks to have worked very well in the past. I have been following the progress you have made on your Columbian Rocks and you have done a outstanding job on them.