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The two measurement systems are not comparable. They both measure thread size but very differently. The "aught" system measures diameter of the thread. So 6/0 is thicker than 8/0.
The denier system is the weight of the thread per 9000 meters. 70 denier thread means 9000 meters weighs 70 grams. 140 denier weighs twice as much so it will be thicker if it is the same material.
Depending on the materials used, the weight will vary so there is no direct comparison between the two methods.
For Uni thread, 8/0 is roughly 70 denier and 6/0 is roughly 140 denier. I guess this could be used as a rule of thumb.
You can usually Google for fly tying thread size charts and find exact measures for most thread manufacturers.
That said.
The aught system is works ONLY within a given manufacturer. There is NO standard for the aught system and manufacturers can and have labeled their threads at a higher "aught" to make tyers think that their thread is stronger and thinner than a competitors thread.
If seeing is believing consider the hook below which has been wrapped with 40 turns of the thread. Compare the Uni 8/0, to the UTC 70, to the Benecchi 12/0, to the Gordon Griffiths 14/0, to the Veevus 14/0 and 12/0. These threads are all about 70 denier polyester and they have the identical bulk and they ALL BREAK within an oz. of 14 oz. tension.
