1. Look at the top factories of that type in your region.

2. Check the top ones wage. If it is 95% or greater then it's very likely that it is the best choice.

3. Check the other factories wages. If the overall wage is higher then the top factories, choose that one.

4. Make sure they don't switch your wage while you are working there. So keep checking your % wage every so often.

Other considerations:
- Region with high medical index.
- Region reserves are there.
If it runs out work in another region for a bit.
- Deep exploration in your region.