Don't force an absolute minimum value for building miners. I've built mineral miners on planets with a minerals value of 40 before. It all depends on the game and what you need. If you have plenty of planets with high values, use low value planets for research and such. If not, you may need more resources.
Monoliths (Stellar Manipulation tech) combined with a Resource Converter (Resource Manipulation) are the best way to build a strong economy. Also, expand like mad. Build tons of colony ships. Then, when you think you have enough, build more. Colonize every planet you can find. The more, the merrier!
