my current configuration:
6.25 => 2.75 (Space), 1.25 (Fn), 2.25 (Shift)
you know that right Shift is 1 column away, you will have to move your hand to use it, right?
I put my right Shift in the split spacebar, and hit it with my right thumb. (before, my right thumb is useless).
though it did take a bit of time to... "transition" from regular spacebar to split spacebar.
my PCB is JC65.
6.25 => 1.25 - 2 - 2 - 1 (the 1u key placement is kind of arbitrary; it could be in the middle, or anywhere among 1.25u modifiers)
The extra 1.25 key on the left would be really helpful. Kind of like the big Command key on Apple keyboards.
7 => 1.5 - 2 - 2 - 1.5
I actually use something like this right now, and the extra large modifiers are very helpful, once again. I really struggle with reaching for Alt on winkeyless keyboards.
IMHO easy-to-reach Alt (or whatever they're mapped to) keys are as big win as a split spacebar imho.
I haven't really thought about switch/stabilizer holes on the PCB/plate, though, and maybe my ideas can't be implemented on a backward-compatible PCB.
good point about Alt key,
before my left hand is placed a bit on the left side of the home key F, so reaching Alt is not a real problem.
now for the split spacebar, my left thumb has to reach the Fn key so I must place my left hand a bit further to the right side, reaching Alt is harder now