I am shooting the Axis 400's with 250 up front in the Buffalo 50@28 to get it to shoot good.  My next arrow order I will get the 500's and try 200 and 250's up front.  I am not sure that I am satisfied with the 400's, maybe too stiff for my style of shooting and setup.

I don't think anyone's charts get you right on--still takes a lot of experimentation.  I would agree with the 400's as starting point with that much weight up front.

If you don't want to put alot of weight up front, then go with the 500.

Keep in mind a stock 29"BOP Axis 500 with stock insert, 125gr. point and 3 5" feathers will weigh 398gr.
With your 50lb. Buffalo you would only be shooting 7.9 GPP of bow weight with this set up. I'm sure the bow would be fine, but it might be a little quieter and more effective hunting with more like 9 or 10 GPP.

I'd get a test kit from 3 rivers.

Cut the arrows to the length you would like to shoot.
Then adjust your point weight up or down looking for a perfect bare shaft flight.

Arrow spine per individual is some what subjective. Everyone shoots just a little different.  

My dad shoots the same bow as I do.  Same poundage and always needs a weaker shaft then me.  

Same draw length too.  Our form, and release is different enough that I need a full higher spine that he does.

Try the test kit, and bare shaft till you get perfect flight.

It's worth it to take the extra time.

Jason

My buffalo is marked 45 pounds and I pull about 28.5. I am getting good flight out of 30inch 400s with 300grn up front. Haven't checked mine, but Hoyt has a reputation for running a little heavy with their limbs.

Full length 500's with the 100gr inserts and 125 gr fieldtips. Good place to start? Maybe good place to finish.