In my short time as a horse owner I have seen a number of ways to give horse grain. There is the 'coffee can' method, measure out 1/2 or 1/3 or an entire coffee can, the 'handfuls' method and the guesstimate method.
In reality none of these are as effective as weighing your grain. Every grain will weigh differently and it is imperative to weigh your grain prior to feeding. Two pounds of oats is not the same as two pounds of senior.
That said, once it is weighed you can figure out what that volume translates to in terms of your coffee can, but don't forget to weight it first and follow feeding guidelines. Some grains cannot be 'overfed' as they will provide your horse with too many vitamins or minerals, so be diligent and take this extra step.
Other weighing tips:
A good digital scale is easy to use and can often be reset to zero, even with your container on it.
A baby scale is a good way to measure hay
A non-digital scale is excellent for varying weather conditions and you don't have to worry about running out of batteries.
A fish weighing scale can be used along with a tarp to weigh hay