@Joe get yourself some calipers man. Cheap plastic ones from the hardware store work great. Or go digital for $9 on Amazon.
Josh Poertner briefly mentioned this and said that you should NOT use caulk because that will be aerodynamically worse. (edit: actually he said it will make rolling resistance higher and that wastes more energy than the aero gains, see below).
Think about it: the caulk is not going to be really smooth and it’s going to create a bit of a lip at the edge on the rim. It’s also not going to be uniform. You are better off just leaving that gap alone.
For the 105% rule, it is a minimum amount that your rim should be wider than the tire, using actual widths measured. More is better, but you can’t realistically get more than about 110% with light weight rims because there’s only so much difference between inner width and outer width of the rim (usually 5-8 mm) and your Tire has to be wider than the rim internal width by a big enough factor or it will roll off.
My rims are 32 external, 25 internal. I have 28 mm tires on them that stretch out to 30 to 30.5 mm due to the wider rim. That barely hits 105%. To get to 110%, I would need tires that measure 29 mm on the wide rim, meaning they would have to be tires that are labeled 27 mm (and actually true to label, which is based on a 17 mm internal rim). But then I would be at higher risk of the tire coming off on hard cornering.
There’s ERTO guidelines for what width tires can run on what width rims. Too wide or too narrow can cause issues. Unfortunately those ERTO guidelines are outdated and too conservative. There’s been work in updating them, but committees are slow.