"realistically", 20% All Might should be plenty, and if Deku only ever gets to 50% All Might in all aspects, a 50% of All Might's top power would still flatten everybody out there.

Remember when All Might said he could have finished Nomu in 5 punches but still took 300?

That puts him at 1.6% of his former self. Deku is technically beyond that now, but I guess you can see why the author isn't making him any closer to All Might. Once you get close enough, you're good enough to win against anybody who isn't All for One.