I see no replies to your inquiry, so I'll address it. You are making concrete volumetrically, so you need to be thinking in those terms. From your calibration information you have the ability to formulate mix designs by weight, but I'd convert them back to cubic feet to get ballpark settings on your volumetric gates.
The high cement mix designs you were quoted will certainly insure that the shotcrete will stick. And I'm asuming that you mean wet method shotcrete and not dry gunite. With the mix design you posted, your sand requirement will be 1,750 lbs. You arrive at this conclusion by knowing that you must have 35 cubic feet of dry material total to make 27 cubic feet of concrete. The reason for this disparity is that finer materials particles fill the voides between the coarser ones, and the water gets inbetween them all. So, until you get up around a seven-bag mix (658-700 lbs), you are going to require 3,000 lbs total of course and fine aggregate in your mix. Allow a rule of thumb of 100 lbs of each per cubic foot. The ratio of sand to stone for simple 3,000 psi concrete by weight is 1:1, or 1,500 lbs of each, or 55%/45%, by volume But, volumetrically, stone takes up about 20% more space than sand for equivalent weight because of particle size and the voids between the particles. Thus, if you want to run out of each material at the same time the stone bin must be about 20% larger than the sand bin. This is probably the way your machine was constructed -- with unequal bins. When high sand content mixes are required, you may have to switch your sand to the larger bin. If you do, make sure it will flow without having to switch the vibrators as well. The stone content was reduced to 1250 lbs to ease the passage of material through the gun, and the sand to 1750 for a total of 3,000 lbs, or 30 cubic feet. You are now going to add just under 6 cubic feet of cement for the remainder of your volume. Because you are compacting the concrete at the surface you are shooting, I would not subtract any volume from the aggregate to compensate for the "extra cement" also, the added water will account for some volume, but is nearly discountable, because it resides between all the particles of cement, sand and stone. If you find you can decrease your cement slightly, raise the sand gate one increment but leave the stone gate where it is. Ordinarily you would adjust both gates, but in this case your stone content is high for shotcrete. If you want to lessen the stone content, raise the sand gate by the same amount you reduced the stone. Also, don't forget air entrainment to help control bleed water at the surface. You might also contact American Shotcrete Producers. Can't remember if that's the actual name of the outfit, but you can find it using "Google". smnstn