I just put my Stout in secondary fermenters to get ready to bottle. My OG was 1.042 and FG was 1.013. It says add .5 to the total to allow for priming sugar. So am I looking at about 4.2% ABV ??
Also, I saw people re-using their yeast. I scooped up all the gunk on the bottom of the fermenter and put it in a container. I put it in the fridge. Do I have to do anything else to it to re-use ?
As long as your container was sanitized, and you have a covering of liquid, then nope you can just add the slurry to your next batch. Assuming itâs an average gravity batch, and you are not waiting months to reuse it.
Using an on-line pitching calculator like âMr. Maltyâ (there are others) will tell you how much of that slurry you need to use.
Was this a 5 gallon batch or 1 gallon. Five ounces of priming sugar in a 5 gallon batch would add about 0.26% ABV.
Keep everything sanitary as @jimrmaine has said. The best liquid to keep over the stored yeast is the beer that remained in the fermentor. I just pour from the carboy into a couple of sanitized pint canning jars. I used to pour into quart jars but just ended up with way to much yeast.
Link to one of the posts on yeast harvesting.
Definitively look into how much to re-pitch from a slurry from a site like Mr. Malty. I finished up a brew using S-04 yeast and reclaimed it from the collection ball of my fast fermentor and dumped the whole thing right into the wort of the other batch I had just done. Walked in my laundry room to feed the cats next morning and the fermentor was foaming over like some expanding foam can was literally exploding on top of the fast fermentor out the air lock. Luckily I always put a catch bucket underneath them. It took about another day or two to calm down.
Lesson learned, only need about ~ 2 oz (1/4 cup) of slurry yeast, not 24 oz. 
Itâs a 5 gallon batch. I usually put sugar in the bottles individually. Wondered about adding to the brew first and then stirring up. Worried it wouldnât be the same in each bottle, or the stirring would cause too much oxygen.
Put my yeast in a âcleanâ container, but not sanitized. Sorry wasnât thinking. 
Oh, man⌠bulk prime, dude. Bottling day sucks enough already, without 50 tiny measurements (unless you do the Domino Dot thing⌠that could work out well).
Mr. Malty is an excellent resourceâŚhowever, 35 years of repitching yeast from batch to batch has taught me that for an average strength brew (ie., in the 1.040-1.050 range) using roughly 1/3 of the slurry is about right (and you can just âeyeballâ itâŚthereâs no need to get involved with decanting to measure it),
The brews I make on a regular basis are of varying O.G., so depending on the beer I have used anywhere from that average 1/3 up to 1/2 of the slurry; for my Burton/Old Ale (barleywine) which I make two or three times a yea, I typically use the whole slurry.
