Supplying common market goods to your country vs world market/ Optimum level

Know people have differing opinions ranging from doing anywhere from nil common market trading to supplying country and sometimes all your corps on the local/common market.

Which strat do you use? I tend to try and supply just my country with goods produced by state corps that are in high demand. Sometimes my corps also get their supply on local/common market. Though I never offer more than 25% of a corps production to the common market. I am not good on the numbers crunching on figuring out what is optimal so it would be nice to hear what others think is a good strat or input on common market trading.

zero common market using it is like playing at a higher difficulty level to obtain a higher score