Is there a simple formula for determining alcohol content without consulting a table?

Yes there is. You can determine alcohol content by subtracting your final gravity from your starting gravity, and multiply the result by 131.

(SG-FG)*131=ABV

For example your starting gravity is 1.060 and your final gravity is 1.012,
1.060-1.012=0.048
0.048 x 131= 6.3% ABV