It’s not a different arithmetic to the one we use in the supermarket, but it is an application we’re not always accustomed to.
Most competition players will know that some sort of mathematical work is required at the last hand of a game to secure your position or try to improve your position. Generally, you shouldn’t be trying to find too much value in your hand if you' only need a quick 1 han win. I suspect that I really need a beginner blog post for that. Today is more advanced…
If you ever take the professional test, you are required to be able to work out what you need to get in all the various possibilities and even perhaps given wider tournament context.
Here is a potential question and its workings to give you a taster.
South 4, West player, 1 honba, 2 riichi sticks
You: 25,800 Top: North 38,000
What do you need to get top if you tsumo?
The raw difference between you and the top player is 12,200. However, we need to consider the sticks on the table to calculate the actual difference.
12,200 -2,000 (riichi sticks) - 400 (300 for the continuation and then another 100 extra difference because the top player will have also paid their share of 100) = 9,800
2000/3,900 (4 han/30 fu) would change the effective situation between yourself and the North player by 9.900 points; 2,000+2,000+3,900 (for the raw score gained) + 2,000 (additional point difference created by the North player having to pay).
You would end with 36,000 and top place. North would drop to second with 35,900. A very tight point difference of only 100! Those continuations can make a difference.
Now try doing that in your head…
Example taken from Mahjong Pro Certification by JPML.