Maybe some people can calculate to the very last grain of sand, but when in doubt, over-estimate.  Have a smaller stone mold or some other small plastic molds handy for use with any over-run that remains.  You will NEVER end up with the exact amount.  Don't worry about something so insignificant.  Just be sure there's enough to fill a mold on the first pouring (and hope you won't need to pave the back yard with the baby blue mix that's left over).  Adjustment gets easier with experience.

I filled an 18" round mold with water and it came to exactly 28 cups.  The formula I use has 9 cups cement and 20 cups sand (2 extra cups for the "pot") for a total of 29 cups.  I usually have some over-run, but the next step down in volume just barely fills the mold.  The total volume estimation can be also be done by filling the mold with sand, but that's HEAVY.  Water works fine and you can just dump it out.

The other ingredients don't add much to the volume of the mix.  To estimate these, you can start with the basic measurements given by Cole on his formula page, or you can save some time and trust that the weeks of bumbling I did yielded some valid information.  If you look at the 3 formulas in Part 2 of the Manual you will see a pattern to the increase or decrease of the smaller volume ingredients as the stone size changes.  For example, there's about a ½ cup difference in Fumed Silica between the 14", 16" hex and 18" stones; 2 tbsp Superplasticizer; 2 tbsp Fiber; CaCl equals about 1/3 cup (up to the max of 1 cup in an 18" round).

By using the tested formula for an 18" round mold we can easily calculate volume for the "Mega" molds.  A 39" x 17" x 3" bench mold uses 50 cups of cement and 100 cups of sand …or a bit more than 5 times as much as the 18" mold.  The new 18" x 4" birdbath mold will use approximately 3 times the formula for an 18" stone.  For a birdbath, just multiply the ingredients for the 18" round by 3 (except for the CaCl which needs to be kept under 1½ cups).  Use a 5 times factor for the bench.

Tree rings and bricks can be done in the same way.  Just find the established amounts for a stone closest to their volume and either use that or interpolate with a factor (plus or minus).  This is probably a math nightmare, but hey, it's concrete, not rocket science.  If you have access to a math teacher or engineer you could get them to calculate the mean cubic volume of a mold by size, then do the same with a 1 cup measuring implement and give you a factor to use.  That's no fun.  Anybody can do that, and you'd still need to know how to adjust the smaller volume ingredients.

Frankly, a bench scares me to death.  I'm in awe of the people who have done them successfully.  I am NOT dragging one into the yard to fill it up with a water measurement.  Thus, an interpolation from an established calculation is much easier and it helps to have verification from people who have tested the large formulations (thanks go to Jess and Chatty).