Experimentation to see what gives you the best results is the key to GTL Ref tuning.

Start with the standard values, and try lowering them, as well as increasing them. See what gives your CPU / board the most stability. You may even gain a few more FSB and MHz as a result.

Some CPU's like lower values like 0.61 - 0.630x, others like higher values like 0.690x, some like the same values on each GTL Ref setting like 0.670x (0/2) and 0.670x (1/3), others like a higher 1/3 GTL Ref than 0/2 GTL Ref, like 0.670x (0/2) and 0.690x (1/3). There are many possible cominations, you have to try them out.

It might help you to write down on some paper the combinations you try, and the results they give. When you have tried all the sensible options, you will be able to see which ones worked best for your gear