Here is how I approach this:
1) put empty reels into tank;
2) fill tank with water, making sure that water covers reels well, with some room to spare, but that there is at least some air above the water and below the lid;
3) pour the water into a graduated cylinder and measure the volume. If it is something like 473 ml, round it up slightly to a convenient even amount - say 480ml. This is the target total volume of all your working solutions;
4) divide the target total volume by 32 (for dilution B). The resulting number is the amount of concentrate to use. For the example where 480 ml is the total volume, the correct volume of concentrate is 15 m.
To actually mix your working solution, you need a small syringe or measuring cylinder that permits accurately measuring 15 ml of concentrate (using the example figures). You also need a measuring beaker or cup that permits reasonably accurate measurement of 480 ml (using the example figures) of working solution. Finally, you need about 500 ml of the water you intend to dilute the concentrate with, to make the working developer. I usually like to use water that is at or slightly above the temperature I intend to develop the film at.
To actually mix the working developer, I:
a) measure the concentrate in the appropriate measuring cylinder, syringe or whatever (in the example - 15 ml);
b) pour that concentrate into the larger mixing beaker;
c) rinse the measuring cylinder or syringe into my mixing beaker using several small amounts of the water I am diluting the developer with, until all of the concentrate has been rinsed into the larger beaker;
d) add enough additional water to bring the total volume of the concentrate and water to the target total volume (480 ml - using the example).
In short, I measure the concentrate accurately, and the concentrate plus diluting water accurately. If I've done it right, I should have a little water left over, which I discard or save for another step.
Hope this helps.
Matt
P.S. it really helps if you can record somewhere what the total volume of the working solution is. Ideally, you should write it on the outside of the tank itself, with some sort of indelible ink.