what is the tax per can of milk powder above 24 cans and how do they calculate the tax.

According to what I know, foreign passengers can take items less than CNY 2000 into PRC. So you can take milk powder that costs less than CNY2000. If you carry more than CNY 2000 milk powder, you should pay the tax. The tax is CNY 20 per kilo, probably a can of milk powder.