Tum Pai actually has a very similar system to WHKD in regards to rank (not surprising, given they have the same founder...). We also have a notebook/binder with the requirements, historical information and fighting principles that is used for note taking and for checking off the students requirements before they test. Additionally, we also have time requirements between dans. At a minimum, 1 year must pass between student black and 1st dan, and only after the student has shown a continued contribution to the style will they earn their 1st dan. 3 years (minimum) are required between 1st and 2nd Dan, 4 more years between 2nd and 3rd, 5 more years between 3rd and 4th, and so on. This has resulted in very few black belts above 1st in the style, and only 30 or so of 3rd dan or higher.