One easy way is to multiply the OG by the gallons of wort/beer and divide by the number of gallons of diluted wort/beer. For example 5 G of 1.065 wort diluted with 1 G of H2O would be 65x5=325 gravity points divided by 6G = 54 or 1.054. Measure the FG after dilution and calculate your ABV from the adjusted OG [1.054]. Hope that didn't muddy the water.
Not muddy, I understand that. But I didnt actually add water to the beer/wort, not sure if I should recalc or not? Its just that I ended up with 6 gallons because I started with 8g of RO water at mash in, which ended up at 5.75g post boil vol, and just before bottling I added a 2000ml starter (for reyeasting/bottle conditioning) which made the whole volume just a tad under 6gallons.
If I recalc as above, for 6 gallons, then my abv drops from 10.5% (OG1.086, FG1.006) to 7.8%. Which is the truer abv?
You can use a basic template for this calculation and just change numbers on future batches. If your added starter was exactly the same original gravity as your beer, you could just assume no change in ABV, but if it was different, you could do this (it is going to be an estimate because who knows exactly how much the starter ferments out - but it is the smaller volume, so estimate is good enough):
New original gravity = (OG of original beer 86 x volume or 5.75 gallons) + (OG of starter - let's assume 1.050 or just 50 x volume of starter 0.5 gallons) / New volume or 6.25
(86 x 5.75) + (50 x .5 = 25) = 494.5 + 25 = 519.5 and then divide that by your new volume which is 6.25 (assuming you added the full starter)
You can plug this revised formula from above directly into Google for it to calc: ((86 x 5.75) + (50 x .5)) / 6.25
OG = 1.083 - this represents the new OG of your beer + starter - again, assuming you pitched a 1.050 starter - if it was lower, replace the 50 in the formula above
New final gravity = (FG original x 5.75) + (FG of starter - guessing 1.008 x 0.5) / 6.25
((6 x 5.75) + (8 x 0.5)) / 6.25
New FG = 1.006
OG 1.083 and FG 1.006 = new ABV of: 10.11% ABV (according to Brewer's Friend)
...and for what it is worth, that is a really, really low final gravity for a beer starting above 1.080.
So... quick recap, you just need to determine what the theoretical OG is when you add two liquids together ((Original Liquid Gravity x Volume) + (New Liquid x Volume)) / New Volume - and then do the same for the FG of the combined liquid. This isn't exact, but it will come close enough!