Don't lose the adaptor for the portable tanks. Soon someone will want you to fill their portable tank for them.
Remember that you can never fill a tank completely. It only fills until the pressures between them equalizes. You will have to refill the big cylinder when the pressure goes below the minimum you will tolerate in the plane. You may only get 3 fills before needing to replenish. With a two cylinder cascade system, it gets better, but you still have a limited number of fills available.
As to calculating the amounts of O2, I don't worry about it. I keep re-filling until it gets down to my minimum and re-fill.
Also, it is very important to remember that when filling, if you hear a "rattling" sound, quickly shut down the tank valve until it stops. That is the sound of the valve on the tank in your plane "bouncing" on its seat, and will quickly ruin it. They are expensive and a pain to replace.
The fire marshal in Dallas, will not tolerate O2 bottles in the hanger unless you pay a yearly fee. When he comes to inspect, my FBO folks tip me off in advance, so I can go bring my cylinders home until after the inspection.