you could try my ultra scientific methods,
fit a diff stem and bars and measure?
Seriously though, im not even sure what you are trying to do...
you want your bars lower and further away... sooooooo
1. longer stem, or longer reach bars are a must
2. lower stem on existing spacers
3. use lower angle stem
see where it gets you.
you ask for an estimate, but surely your own eyeballs can give you a better estimate of something in front of you than extrapolations from a bunch of yahoos on a bike forum?
what is the ultimate goal, its hard to help when im not sure what you are ultimately trying to do.