Ratios will vary, probably a lot.
I think that helmets became optional in Florida on July 1, 2000, for those
over 21 with sufficient medical insurance (I do not know the criterion).
I have not been studying the use of helmets, but I imagine that many of those who have foot controls well forward of their butts go without helmets, while most riders whose feet are basically under their butts wear helmets.
There are exceptions. My neighbor has a CBR-1000 and wears no helmet.
I visit Sarasota's Fuel Cafe, a genteel hangout for bikers, mainly riders of sporty machines, and probably 80% or more of the visitors to that place wear helmets.
I have ridden with Stromtroopers in several provinces and states, but Florida is the only one of those places where helmets are not compulsory for all. Nevertheless, I very very seldom see anyone lidless on a Strom, and I surely have seen a lot of Wee-Stroms.
I am glad that helmets are optional, because every safety device includes a hazard, and I consider it fair to let those who ride decide what safety devices to adopt, along with their accompanying hazards.