Well the 12-unit grid is normally chosen based on being divisible by [1,2,3,4,6,12]. 16-unit is divisible by [1,2,4,8,16], but I see that you added a "thirds" class to deal with that. 20-unit could be an option as it covers divisibility by [1,2,4,5,10,20] but also critically ignores 3 column. 24-unit (the product of the first four positive integers) is divisible by [1,2,3,4,5,6,8,12,24], so it would naturally allow the most flexibility at the cost of very granular 1/24 elements.