I think the finger diagram thing is for general placement. I kinda use the finger placement. but have gotten used to doing it a bit different. For example, my right index finger is the one usually pushing the space bar. I'm not at 70WPM though, kinda hard to break habits though.
Hmm, space should be from either thumbs. What else will you use them for?
Thumbs are competely useless for typing other than space. This particular habit is easily broken... Rest can be difficult. Because index finder and space means you have to twist your hand a bit. Feels awkward when i tried just now lol.
I used some modifications too, but as i grew and my hands too it kind of corrected automatically. Never made effort for it.
I am actually average at 80. Highest score i have is 100. All fingers correctly used.
But this limit is difficult to break.
90 if i type from memory and 60 if there are to many punctuations.