I would think that the stats are impossible to know with any confidence beforehand. My recommendation is to ball-park it around 34 ppg and maybe a 50-60% extract efficiency, calculate the recipe around that, but keep some extract or other sugar source on hand in case you fall short of your target OG. That's how I do it, anyway. I take a gravity reading after lautering, adjust for temperature and boil-off, and then figure out what my efficiency was based on an estimated 34 ppg. Then I calculate how much sugar/extract/whatever to add to the boil to reach my desired OG.