Normally you add the curve to your results (or subtract). So for 1kHz freq you let it as it is since it's reference (0dB in the table) and then, for 2kHz you add 1.2dB to your results, for 40Hz you subtract 34.6dB, etc. Following a table you can find anywhere, usually in 1/3 oct or 1 oct bands. Easy to build on exel or similar. Not much there. Once you have all your values you add as usual with dB. Take level from dB, sum=10^(20Hz.lvl[dB]/10)+10^(22Hz.lvl[dB]/10)+...+10^(20kHz.lvl[dB]/10) and then 10log(sum)=A-w.lvl[dB]
Notice it's always with 10 and not with 20 or squared since you want to add already squared, as power, not voltage or energy, not pressure. No need to use the reference, since all will keep the same, divide and then multiply once summed, no reason to do that unless you want the energy or power involved but doesn't make sense since it doesn't has a physical meaning, once A-wighted.
JS