Royal, in time you will know when the consistency is right just by swiping your margin trowel into the mixture and feeling the resistance. Practice is all it takes to get that "feel".
Now the question becomes, how to get to that point. If the mfg. ratios seem to yield a mix too dry, try bumping up the water by a few percentage points or ounces, whatever the case may be. Caution, towards the end of the mixing process a little water goes a long
Mixed right the work involved to pack it into the spaces should not be hard. Mixed right the grout will "move" with the float and into the spaces. Mixed right, tip the bucket over 45 deg. or so and the grout should begin to move or slump but not run out. Mixed right, I'm betting for most grout and environmental conditions the water to dry volume percentage is going to be in a range of 25 to 30%.