Good morning & thanks for any advice. First post as I'm a new Martin owner. After shooting 11 bows for fit & feel, I got a Cheetah a couple weeks ago.

I bought some Easton Carbon Arrows at Dick's, 9.5 GPI, and put on 100 Gr. field tips, then went to a friends house who owned a chrono. We found that I shot a full 30 FPS faster with his "Flatline 500's" that weigh 6.5 GPI. That sparked several days of confusing research that I'm hoping someone can simplify a bit for me.

In order to get a few more FPS without blowing an arrow apart/through my hand, how light an arrow can I safely shoot at 29" draw, 70#, 100 gr. tips, arrows cut to 27"?
And what's the critical measurement..."Grains per inch" or the "200/340/400/500" info found on the charts on the arrow boxes?

Thanks so much for any help!!!