So if we are going by the example stat spread, if I put my MA as 60, my Rokushiki mastery would be 60?
No.
There's main stats and substats.
Main stats are strength, speed, vitality, MA, DF, Mind, Haki. (you don't necessarily have to invest in all of them, for example you don't need to have a DF or any Mind (this stat is for specific arts, it doesn't make your character dumb if you have 0 on it, it's just for stuff like medicine, science, navigation, etc)
And then some of those main stats have substats.
So the main stat speed, splits into two substats, reaction speed and movement speed.
The main stat vitality, splits off into stamina and durability.
Those main stats sometimes have a multiplier to calculate the points you get for the substats.
So for example speed, vitality, MA and Haki have x2 multipliers.
So if you put 40 into speed, then you get 80 points to split between its substats, reaction and movement.
Same with vitality, you could put 50 vitality, so you would get 100 points for its two substats, so you could for example put 40 into stamina and 60 into durability.
With MA, it also has a x2 multiplier, so if you put 30 into MA, you get 60 points to put into martial arts. But, you don't necessarily have to split it into two martial arts, you could put the whole 60 into just Rokushiki.
Keep in mind there's stat caps, a level 50 can put a maximum of 75 into any stat category.