you can't "get it" to a certain HP. You need to be born with it... either you have it from birth, or you don't. If you play on the real gameboy, then don't even bother with it, as the chance to get a satisfying HP is as low as..
1 divided by (31^5)
Don't forget that you'll want a certain nature too, which is 1/25 on top of that, so
1/(31^5) * (1/25)
Winning the lottery is perhaps just as difficult as getting HP Ghost 70... the equation isn't totally correct, as there's more than one method to get HP Ghost 70. This is the equation for HP Dark 70, where there is only one way to it... but it's not off by that much at all.