To target an ABV you need to have a good estimate of your yeast's attenuation. Then you can do some algebra on the yeast attenuation equation and the ABV equation and solve for a target OG.
Formula for yeast attenuation is:
Attenuation = (OG - FG) / (OG - 1) [multiply by 100% to get % attenuation]
Formula for ABV is:
ABV = (OG - FG) * 131.25
If we solve the ABV formula for FG, and then substitute for FG in the Attenuation formula, we can then solve for OG:
ABV / 131.25 = OG - FG
FG = OG - ABV / 131.25
Attenuation = (OG - (OG - ABV / 131.25)) / (OG - 1)
Attenuation = (ABV / 131.25) / (OG - 1)
OG - 1 = (ABV / 131.25) / Attenuation
OG = 1 + ABV / (131.25 * Attenuation) [where Attenuation is fractional, not percentage, 75% = 0.75]
We can use the last equation above to determine our target OG from our desired ABV and the yeast attenuation. Let's do an example. If we want 4.0 ABV, and our intended yeast has an attenuation of 75%, the calculation looks like this:
Target OG = 1 + 4.0 / (131.25 * 0.75) = 1.0406
Target FG = 1.0406 - 4.0 / 131.25 = 1.0101
The above target OG is the SG in the fermenter(s). If you want to brew a higher OG batch and dilute it into two lower OG batches with larger total volume than the brewed batch volume, you have to figure out the target OG for the brewed batch. In that case the math looks like this:
(Brewed OG Target - 1) = (Fermenter OG Target - 1) * Fermenter Volume / Brewed Volume
Brewed OG Target = 1 + (Fermenter OG Target - 1) * Fermenter Volume / Brewed Volume
Let's say you want 5.5 gal of 1.040 SG wort in each of two fermenters, and you can brew a 6.5 gal batch. The total fermenter volume will be 11.0 gal, and the target OG for the brewed batch will be:
Brewed Target OG = 1+ 0,040 * 11.0 / 6.5 = 1.0677
Brew on