Could you measure your foot and let me know what size you purchased? Does your toe touch the end of the boot?
Sure. My foot measures an even 10". I added .75 inch to that for the "toe clearance" Forma talks about in the photo below from the Forma website, and according to the Forma sizing chart, I should order a 42, which I did. (I did that last fall when I got my first pair of Forma Adventure boots, the high ones, and they fit great.)
My feet do NOT touch the end of the boot. You don't want them to; that's why you add the .75 inch to your foot measurement- to give your toes room, like you would in any shoe. Plus, from what I read from people who have spent much more time on a bike than I have, your feet will swell as the day goes on, so you definitely want to leave yourself that .75 inch buffer.
Your math is correct; according to what your foot measured (11 3/4"), and the size chart, adding 3/4" to that = 12.50, which means you should get a size 15 (which I see that ebay seller doesn't have). If he was taking returns on that boot, you could try a size 14, but a 14 will probably be too small on you. Keep in mind the socks you'll be wearing, too. I wear merino wool, on the thicker side, so... I need that 3/4" of space.
One suggestion is order a size 15 from Revzilla or another place that would take a return, so that you can see which size fits you best. Then, keep your eye peeled on that ebay seller's site for boots in your size. I checked his site- there are a LOT of motorcycle boots for sale.