You're right regarding 65%. The extra 15% is the arrow keys, Page up/down, home, end and delete keys. Found I often hit these one handed while the other hand is on the mouse, so a secondary layer that would require two hands to activate would be problematic.
There's neuroplasticity and there's picking your battles. :-)
I had my own set of keys and combinations I wanted to hit while mousing, mostly space and cut/copy/paste. I just made sure I had those on a layer activated by my left thumb on my left hand.
My layout would not work for you since I put navigation on the right hand (close to where it would be normally), but do whatever works for you.
Personally I use navigation keys much more on my split because they're right under my fingers. Having VIM movement keys everywhere is quite nice.
As for <60%: use that neuroplasticity. Stave off dementia. You can do it.