I'm not sure I follow you. You use (your extract)/(extract potential) for your efficiency... extract potential depends on the recipe and grains used.
Or are the extract potential values you have for different grains given in ppg or something? I think that's points pound per gallon or something. That's points of gravity per pound of grain for a 1gal batch. So if you wanted points per kilo per litre or something, it shouldn't be too hard to convert. So if you get 1 point from 1 pound of malt in a gallon of water, then you get 1 point from 0.45kg in 3.79L. Notice you'll get higher gravity mashing the same weight in less water, so we multiply the gravity by 3.79 and get 3.79 points per 0.45kg in 1L of water. Then again, mashing more grain will give you higher gravity, so we divide by 0.45 to get 8.422 points per kg per L.
So without all the rounding, 1p p g = 8.3454 p kg L ...I think :unsure: