Hi Sept. measuring up the back of the heel will always result in a higher measurement unless you have a straight cut heel. That’s why you should measure on the side as shown above. The more forward curve of the heel, the greater the discrepancy will be. That is why the industry has developed a standard for measuring. BTW: 120 mm is 4.72 inches. (2.54 cm per inch or 25.4 mm per inch.) From what I can estimate from your picture, your size 35 is actually about 118 mm or 11.7 cm. Forgive me for sounding pedantic, but I worked in industrial design and can’t help myself😉.