Well I think the point is that if you just do double washing, you're still only doing 5 points into HP per level and that won't make it to 30k. (even though using double you would gain 78-90 MP per level, you can only wash 20 of that out with 5 AP, so you have plenty of MP to spare)
If you do STR>HP, then MP>STR, you get the additional 22 hp per time, and can make it up whenever. It's just expensive.
If you do STR>HP, then MP>STR, you get the additional 22 hp per time, and can make it up whenever. It's just expensive.