To your first question: you should limit the bulk charge phase to about 25% of the battery capacity. Some sources say 20%. So 50 amps is way too high. It will cause the batteries to overheat and start throwing off hydrogen and oxygen.
Second, once the batteries reach about 80% of full charge, you should switch to a voltage limited absorption phase. As the voltage of the battery and the voltage of the charger approach each other, the amps taper off. Kind of like Terry's two milk jugs, connected by a tube. As the level in the two jugs get closer together, the flow rate through the tube will slow down. Likewise, as the voltages get closer together, the flow of electricity will slow down.
Regarding the math problems. Because of thermal losses, you need to put about 20% more energy into a battery than you take out. So if you take out 50 ah, you need to put back in about 62 ah.
Battery charging is a non-linear function. It can get very complicated if you start to look at different temperatures and rates of charging and discharging. Then there's the 'Peukert' factor, which says the faster you drain a battery, the less total energy you get out.
A battery is a storage device, but it isn't just a bucket that you can put in and take out at a fixed rate. Going back to the milk bottle analogy, think of the battery as a bucket that you are trying to fill to the very top. You can fill it very quickly when it is near empty, but as you get toward the top you need to slow down or it will splash out over the top.
Here's a website I found useful: http://www.pacificpowerbatteries.com...AQ/dcfaq6.html