My 51# likes 60-64's and my 60# likes 70-74's, both at 29" bop with a 125 grain point. The 10-15# over works well on mine but I draw 29" so I think that with you at just under 28" draw I would look around 5-10 over which is right in the 60-64# range.
Two things to consider is that the '56 is not cut to center and you draw under 28", both traits will usually want a softer spine arrow. Both of my bows really like arrows that weigh 10 grains per pound as well.
I agree with moleman and Bjorn, 60-64 is a good starting point but you never know where you'll end up until you try a few...