there isnt really a correct way to tighten your timings, different methods work for different people and different systems. I don't see why you only want to lower your CAS Latency, a combination of all works best. To tighten the CAS latency just lower the timing by 1, if you manage to boot keep lowering by 1 until you fail to boot. Once you manage to boot with the lowest timing then run memtest to check if it is stable. You could go tighten the CAS even further by increasing voltage but I wouldn't recommend that.
In my opinion you should try a combination of all 4 timings. Try lowering the 1st and 3rd timings by one, if you manage to boot into windows then lower your 2nd timing as far as it can go without failing to boot. Now lower the 1st and 3rd timings again to as low as they can get with the 2nd timing tightened to the max. Once you are done with those you can now lower your 4th timing. The 4th timing should generally be the sum of the previous 3 timings, so say for example you managed to get timings of 7-7-7 for the first 3, then a good starting point would be 21 for the 4th value, although there is no guarantee.
Good luck, hope this helped!