As Gringol and Bjorn point out, a normal 9-inch taper one arrow diameter takes off about 20-25 grains of weight depending on original diameter and material. That translates into about 1% FOC, so at it's best, going to 1/4-inch will add about 2% FOC.
As a very general rule of thumb, I've found that I've had to add about 5# of spine for every 50 grains of additional point weight over 125 grains. So, if you're going up another 100 grains in point weight, you'll need at least another 10# of spine, maybe more. Keep in mind that as you add spine weight, you'll often be adding additional arrow weight, which requires additional spine, etc.
Good luck.