That percentage is when it doesn't mater which hitbox you hit with, it can start working at 50 or even slightly less if you hit with an earlier hitbox, but not too early.
Its not mario specific, just add 10% for snake, minus 5 for falco and so on.
Also note that after 70%(mario, character specidic) you will have to run for a very short time for a sweet spot. At higher percentages you have to walk a tiny bit for a point blank throw.
You don't need the sh turnip dthrow, point blank glide toss to usmash works too, weak bair just a nice setup.
I made a guide on this about a week ago.
on a side note:
After watching the diddy footstool infinate, i figured peach can do the same at higher percentages with her turnips, with the aid of her float. at lower percentages she can combo with dthrow turnip to insta throw dthrow turnip. I've been checking frame data and it appears to be legit.
this is just a teaser, i'll post the specifics later.
Also found a safe shield combo that i'm not gonna reveal yet.