The only way the hybrid line can become P1 is by going through the entire five or six generations. Using a F1 over a new line does not mean he is a P1. He always an F1, & is also now carrying two sets of recessive genes which are going to express themselves in his children & grandchildren, however breeding an F1 back to a new P1 line will give you the hybrid vigour the same as a F1, but in the children & grand children you have the recessive genes from three different lines to sort through. I wouldn't breed the F1 back to new P1 animals unless I had a very good reason, three groups of recessive genes & at least one more generation till reaching P1, is it worth it? I'd just put this investment into having as many F3 animals on the ground in their generation to select from, if you don't have what you want use the F2 again & again untill you get the animals you are happy with. You must realise that the F3 is where you get to be a breeder more than any other generation. Anyone can cross P1animals & get magnificent F1 hybrids. It takes more to produce a new true breeding phenotype. A mule deer/whitetail cross is good for this discussion. What are your objectives for making the cross in the first place?