Because you're using the same aircraft stations every time, it's an easy calculation that you can do on Excell or any other application that can add or subtract.
You can also get commercial software such as
http://eflite.com/software/aircraft_...lators_std.php
www.fltplan.com has a weight and balance feature, but you have to build the profile. A lot of aircraft are in there already, but not the Skymaster. Be the first to build one yourself, and then it's there for everyone to use or modify for their own use.
I actually prefer the American Aeronautics plotters, and have used them on numerous light aircraft, a lot of medium turboprops and turbojets, and some faster, bigger equipment, too. They work great. I've used them under Part 135 a lot, and the FAA likes them. They're easy to use quickly, to use for multiple legs in the ame airplane with differing fuel or passenger loads, and if you order their duplicating forms, you'll get two copies with each weight and balance; one to leave behind or forward to your agency, and another to keep as a permanent record. Very handy. They come in aluminum and fit neatly in a flight "can" or your flight bag, and last forever. You can have them custom made for your aircraft, or accept a standard one for your make and model.
http://flyincg.com
They sell the ifly software, or the American Aeronautics plotters.
The nice thing with the vector graph and plotter method is that you can clearly see from the graph where your CG lies, where it will go with fuel burn, if you're in CG with your empty weight, zero fuel weight, loaded weight, and as you add each items, and it's very easy to see what you need to do to the load to bring it in CG or keep it there; you can plan out the entire trip that way and see exactly what fuel burn, or taking on or deplaning passengers or cargo will do to your load, where to put it to make the CG work out, etc, immediately and visually.
If you don't want a permanent record, you can get the laminated graph and draw on it with a grease pencil, too. Very economical. I recommend keeping a record of your CG for each flight, but it's up to you.