Size 3 Thread Substitution

I got a question yesterday from a customer in Australia who had purchased my "English Country Cottage Cozy." She wanted to know what the UK or Australian equivalents to size 3 and size 10 cotton are (both are used to make the Cottage.)

Yikes! I had no idea what to tell her, so I quickly did some research, which unfortunately, was not much help. I did find out, on a site called "Crochet Australia," that our size 10 is the same as UK and Australian size 10. But I could not find any info on the size 3.

I remembered hearing once that 2 strands of size 10 cotton are the equivalent of size 3. So I decided to make a couple of swatches. Here is what I came up with:

The pink swatch is 2 strands of size 10 held together as one. The white is size 3. Both swatches were worked with the same size 0 steel hook.

I think they are close enough to say the two strands of size 10 make an acceptable substitution for size 3 (at least for this pattern, where gauge is not critical.) They are actually even closer in size than they appear in the picture. When I place one on top of the other they are almost identical; the pink is just a hair smaller.

But does anyone know where I can find the information on crochet thread size equivalencies for America, UK and Australia? I have sold several patterns to customers in other countries, and I want to be able to help them if needed.