The Drexel stuff is what I tend to get.....but I look for 41% glyphosate first...
How much you use will depend on different things. All those pages on the product label is where all that stuff is at. Spot spraying typically is roughly 2% (2.7oz/gallon), while boom type spraying can be as little as 8oz per acre to as much as 32 or 48 oz per acre, while stump treatment or basal spraying is even a higher concentration. It all depends on the weed, how mature it is and the application method.
Now, I will be honest. I cheat, because I am lazy. I use a 2%+ solution all the time and it seems to handle most of what I need. I mix 1 1/4 gallon of gly into my 55 gallon sprayer and spray away! I typically buy a 2.5 gallon jug of gly and I simply divide it in half between 2 tanks/trips. I am sure sometimes it's too hot and sometimes it's not as strong as it should be. It however tends to do what I need or at least a good enough job. There are several variables that come into play when spraying a large area (nozzle rates, number of nozzles, travel speed, spray width.....). The only time I figure it out down to pretty darn close is when I am dealing with a far more costly chemical. Gly is about the cheapest chemical I buy ($32 for 2.5 gallons is pretty cheap compared to some). Some will look down upon my methods, but I live on the property, the chemical is cheap and I can get more gly in a 30 minute drive and be back spraying if I need to.....shoot me.