I set up a template with a line spacing of 1.15x and the line spacing drop down lists it as 1.2x even though it is in fact 1.15x as I can clearly see the text shift if I set it to 1.2x.
Which version are you using? If it;s the v. 3 Beta, you should post it in th Beta Testing forum where the devs will see it. But it will always help if you include information about your version of Windows and the Scrivener version.
Mark
I have the same issue. I use Scrivener version 3.4 for macOS. When I set line spacing to 1.15, it rounds to 1.2. The Windows version lets me enter 1.15.