I have found that because wood is spined on 26" centers, it is necessary to add 5 lbs for each inch of arrow over 28". Add 5 lbs/25 gr increase in point weight. Add 5 lbs for Fast Flight and deduct 5# for traditional longbow.
At 70# @ 31" 70-75# spine, your adding 15 # for 3 inches, and 5# for added point wt., minus 5# for traditional longbow. Nothing added for FF string, assuming you are using B50, D97. It figures out to 85 - 90# spine with the 160 gr. maybe 80-85 lbs.
You might try shooting a few shafts that are heavier spine. If you can't find some, let me know I have some you can try, bare-shafts.
It helps to shoot arrows of the proper length, if you have more than 3/4 - 1.0 inch overhang with a broadhead, think of it as wasted. You gain nothing.