Grinding Trading Skill

Most of us will think buy more will get more skill proficiency. Well its wrong.
This is the math for grinding your trade skills efficiently:

20 x base rank (your rank without booster)

If you buy more than that math allowed, you will waste skill proficiency.
Now how to do it?
Example we want to grind mineral trade. All you have to do first is find city with the most mineral trade goods. I choose hamburg.
What we have in Hamburg are:
 
- lead ore 100
- iron ore 120
- copper ore 123
- tin ore 78

This numbers are example only.
If you have rank 5 Mineral Trade skill, then based on the math, you should buy 100.

1. Buy 100 lead ore, and then click OK
2. Then buy 100 iron ore and click OK again.
3. Buy 100 copper ore...

Now you will see that hamburg still have 20 iron ore, 23 copper ore, tin ore 78. you can mix it as long as its the same class of trade goods.

4. Buy 20 iron ore, 20 copper ore, and 60 tin ore, then click OK.
5. Buy again 3 copper ore, 18 tin ore, then OK again.

Try it, and you will see the different. ^^