any advice on getting a correct gravity

Can anybody give me any advice on the formula for getting my gravity. I check the og after the boil and before I pitch the yeast we pour some in the tube that came with hydrometer and check the temp then the og. Then I check the final gravity at the end of secondary before I add the priming sugar. Then you subtract final from og and take what's left and multiply it by what number? We had the number but I can't rember it right now.(Wasn't shooting for a specific gravity so that's why I took it before I bottled and didn't check it earlier to get it at a certian gravity). The product of the formula kept coming out to like 13% and I know that's not right. Tried it three times and the og was always the same and it was all different batches. It's a dme base with a mini mash grain sac bag if that helps. The beer almost always turns out good and 1 just took second place in a home brew comp so it taste good just frustrated in having a hard time figuring the gravity out. If you have any advice it would be greatly appreciated! Thanx cheers!

I am not that smart. Here ya go!

http://www.brewersfriend.com/abv-calculator/

I think you are looking for the ABV (alcohol content)? To get that take your OG which is your wort in your fermenter before adding yeast. Then take a reading after fermentation is complete which is your FG. Take the difference and multiply by 131.

So if your OG is 1.050 and your FG is 1.012, you have 1.050-1.012=0.038*131=4.98% alcohol