HP Washing

You just need to pass 10. Since the MP you get is INT/10, but rounded down. So if you only have 9 INT, then you'll be at rounddown(9/10) = 0 MP gain the first level. Afterwards you go 9, 14, 19. Meanwhile if you started at 10, you'll go 10, 15, 20 and get that extra MP at level 3. Eventually it adds up (I'd say).

Once you're done washing completely you reset ALL your INT back into your main stat using AP resets (which is why its a costly process). Imaging using NX to HP wash:

1. Adding Level up AP into HP
2. Using an AP reset to -MP, and then +main stat
3. Doing this until your goal

AND THEN you have to use AP resets to -INT, and then +main stat. Imagine having like 120 INT that alone is a decently long process. BUT the end result is that you don't lose anything in the long run~

It's just that its an annoying process that impedes your damage in the short run, but doesn't affect you in the long run