Is it an older Kmag with B-50 Dacron or newer with fast flight. If newer with fast flight I'd bump up to next spine group. In my 1962 Kmag, 45# drawn to 28.5, I'm using arrows spined 50-55 with 125 grain points. This is with a vintage B-50 endless string made by Bear.
I have accumulated enough different bows and arrow sizes that I can test an arrow type and spine. That being said, if you're at the arrow length you'll be shooting, you can always add a little point weight to the arrows you buy (with wood arrows you can add to the front of the broadhead taper if needed) to weaken it, but you can't make a wood arrow stiffer without shortening or adding weight to the back. Also, if you're planning on servicing wood arrows for several bows, investing in a test kit of the arrow material of choice is probably a wise investment.