How strong ?

Hi all I cannot make out how strong my beer is likely to be ?
My starting OG was 40 ish and looks to be ending at about 15/13 but I'm lost on the math I'm guessing about 3 ? Any help would be great !!

3.53 abv at 1.013fg
3.27 abv at 1.015fg
Calculated at 1.040 og

Cheers have you got the method as mine got sanitised at some point and what I found on the Internet made less than no sense

(OG - FG ) x 131.25 = ABV%

Some guy dusted off is algebra book and came up with a formula that IMO is fairly accurate, albeit somewhat annoying to remember. I think it's more accurate than the (OG - FG) x 131.25. The only caviat is that you must use the whole number of the gravity reading (i.e. 1.056 is just 56 in the formula). Anyway the formula is below:

((832+OG) * (832+FG) * (OG-FG)) / 5,500,000 = ABV

For instance:
OG = 1.060
FG = 1.010

832+60 * 832+10 * 50 / 5,500,000
892*842*50 / 5,500,000
37,553,200 / 5,500,000 = 6.8% ABV

7.2 sounds way high for a London ale

My final was 1.01 with a starting of 1.04
So I'm reckoning that 3.92 abv is closer

So bottled half and easy kegged the other half
Just hope it's drinkable come Christmas

Cheers I still think under 4 sounds more likely

But anything s possible I over hopped if that's possible

Well it's only my second beer and I put the hops in for about twice as long so very bitter and dry