Depends on how much of that molasses flavour you want in your end product. In a 1 gallon batch, using molasses as the ONLY added sugar, you might start with 2 - 4 fluid ounces of molasses. Most of it will ferment out, but there will be some residual flavour; You'll just have to experiment as to how much added molasses will give you the flavour profile you're looking for. I can't taste 3 oz in 6 gallons, but I can sure taste 8 oz in six gallons. I use Grandma's Original Molasses:
You might want to use either cane sugar or FAJC (thawed) to boost the ABV & just use the molasses as a flavour adjunct. The point being that you have Pretty much everything except the molasses flavour profile & with that acheived, you can always add more molasses until you reach the desired flavour.
That's the best info I can give you, as I've never used molasses in a 1 gallon batch.