Replying to Mounting without exact BSL

Hi All, I have some questions about mounting points and I was hoping you could help!
Essentially, I do not know my exact BSL as my boots are in the Alps. I am about to buy some Salomon Stance 96 along with the Salomon strive Gw 12 Bindings. They offer a free mounting I was wondering how much it would affect the mounting If the Mondo point was off by 1. My UK shoe size is 7.5 so I imagine my Mondo point is the low side of 26.5 for my tight boots. The BSL for my Head Vectors at this length is 305. How much will it affect the mounting position if they are a size down 293? How much affect is +- 6mm on mounting? If I am shoe size 7-7.5 will the MondoPoint be 26? Is it worth just trying to do it for a price in the Alps and wasting some ski time?

Thank you very much!