Im suprized that you missed the fact that if you cast it, take 2 hits (out of the theoretical 4) and then renew it...it only lasts the remaining 2 hits, not the 4 it should.
You have a friend with prot spirit and bond on him
He gets hit for 300 damage
150 is redirected to you
150 to him
150 is reduced to 50ish from prot spirit for him
150 is reduced by that damage absorbtion number on prot bond for you so you take roughly 125ish damage
I think the guild wars system applies damage reduction last. It goes through the skills and affects and redirects first then applies damage reduction.