Fred,

This is a tough one - as there is a lot of variation due to other factors aside from the extract fermentability.

For example: yeast, temperature, boil time, mash methods used, aeration and lots of other factors all play a part.

However, it is certainly not impossible at all - but so far I have found no verified equation that takes into account the extract fermentability plus contributions due to other factors (yeast attenuation) plus the contributions from steeped or mashed grains. If you know of this kind of research/equation, please let me know as I am very interested!

Cheers!

Brad