Incrementation by Seven
Just count the numbers on the cycle, e.g. 0-7-4-1-8-5-2-9-6-3-0-7-4-1- etc. Know this by heart before you do step 2.
Do first as beginner, then migrate to slow, then medium, and then fast. Keep up in counting along with the graphic progression.
Remember, a 0 means any number that ends in a zero, like 10, or 40 , or 2430.
This is maybe the most difficult routine to master.