UI Scale is one way, but when I'm making an addon, I can't set the users UI scale to my value. So I have to make a workaround. If I scale any value with the following function, there comes out the value needed to represent the amount of "virtual pixels" needed by the frame to get the amount of "real pixels".
Code:
local mult = SetCVar("uiScale", 768/string.match(({GetScreenResolutions()})[GetCurrentResolution()], "%d+x(%d+)"))/GetCVar("uiScale")
local function scale(x) return mult*x end
someframe:SetHeight(scale(10))
someframe:SetWidth(scale(10))
I implemented this method in TukUI and ncUI to get them both pixelperfect.