I don't even bother looking at the numbers on the side, here's what I do.
Get an empty 1.5 litre bottle of Coke and cut the top off it at its widest point, strip the plastic wrapper off it so you can see everything in it, wash it out first then fill it up with water, add a bit of washing up liquid and with a spoon skim off the foam so you just have the water in there. Then shot your float how you think and drop it in the water and see exactly how you're shotting it, this just takes away the doubt especially if you shot it exactly then use a piece of luncheon meat which will over shot the float, in those cases you want a bit of the body sticking up aswell. Keep the line short and bulk the shot under the float, too much line will touch the bottom of the bottle and give a false indication and if any shot is resting on the bottom of the bottle that too will give a false reading.
I do this for every different float I have, if you have a few of the same float pattern and size then you'll only have to do this once. Its much easier than guessing at it and having to strip shot at the bankside when its over shotted.
If you're hyper organised then write down a record of each float and what you're favourite shotting pattern is for each one then you won't have to rely on the numbers on the side or have to use the coke bottle every time you do a new rig.