I have always struggled with the proper shoe size as I always felt like I'm in between sizes. How much space do you leave in front of the big toe? My foot length is exactly 27.5 cm. I go with size 11 and have about a thumb width between the toe and the tip of shoe.
I sold tens of thousands of shoes and boots in my younger years when I was working my way through college, etc.
Point to be made: please don't be so OBSESSED with having a "correct" amount of space between your toes and the end of the shoe! There IS no "correct distance" but so many customers came in ready to measure it with a freakin' micrometer. It depends on the shape of your toes (flat across the tips? curved? Big toe an inch longer than the others?....), the width of the toes and/or ball, the shape of the shoe's toe, all kinds of things. The main thing is to put the shoe on, tie it and walk back and forth in the store. What you want to look for is
plenty of toe room; that is, room to freely wiggle your toes. For me, that's usually about an inch from the tip, but again, that distance means virtually nothing. It could be half an inch, it could be 2 inches. The shoe should support the rest of your foot well enough that you don't move around in it, especially when stopping forward movement, but do so without feeling tight, like it's squeezing your foot.
Also, don't be so obsessed with sizes. A 11 in one shoe can be tighter than a 10.5 in another shoe, especially if it's a different brand. In western boots, it's even more pronounced, with dramatic differences between the same boots in the same size. I would always do a major eye roll when a customer came in declaring "I'm an exact 9.5." There's no such thing. Trust me.