It depends on your efficiency, the type of extract, and the wheat extract ratio.
Let's tackle this in 2 parts. First, barley malt only, then the more complicated wheat extract.
For a normal barley extract, assuming you get 75% efficiency, you will get about 36 x.75 = 27
pts per lb from your grain. At 80%, you will get 36 x .80 = 28.8 pts per lb.
For lme, you will add 36 pts per lb
For dme you will add 44 pts per lb
So, just figure out how many pts of gravity from base malt you need to replace, multiply lbs of grain times your efficiency times the max extract of the grain (36 usually is a reasonable guess), and then divide that by your extract type pts per lb.
(base malt lbs x 36 x eff) / extractPts = lbs of extract needed to replace the grain
Now, Wheat malt extract is usually a 60/40 or 50/50 mix. So we need to take away some base malt since there is both wheat and barley in there:
Let's say you have a 60/40 extract ratio like Briess. That means for every lb of extract, you will get .6 lbs of wheat and .4 lbs of barley malt extract.
(Let's call those numbers the wheat ratio and barley ratio.)
We then calculate the new grain bill in 2 parts:
(wheat malt lbs x 36 x eff) / (extractPts * wheat ratio) = lbs of wheat malt extract to add
(lbs wheat extract * extract gravity pts * barley ratio ) / (36 * eff) = lbs of base malt to remove.
(Somebody else please check my math. I have to go take my kid to school)
