I can't understand the position attribute's scaling on jit.gl objects

I'm using the jet.gl.text2d object to draw text and the position attribute seems to be measured in something arbitrary that I can't understand.
I get that it is relative to the size of the jit.window but the text gets out of the frame when the x position is around 0.9 which seems a bit random, and even that varies when changing the jit.window's size.

Could someone please explain to me if there is something that I don't get right?

Ultimately I would like to make a margin for the text that won't be relative to the jit.window's size.