Sounds like a question for..... slowtwitch! (bike fit question)

I have a new roadbike. Fork is uncut. No spacers on it but I can move it up and down to determine where I want it.

So… how do I figure out where I want it before I get the fork cut? I can ride safely on my trainer or computrainer to figure out fit but what guiding principles should I use?

On my tri-bike, I use no spacers and am all the way down. Fit is really good (I think).