The can method is woefully inaccurate.
Even if you don't take into account the different penetrative capabilities of different weights of BB, different plastic configurations and other material differences, you are still left with the thickness of the can material, how it is treated, different materials of construction etc. etc.
How far something penetrates is never a good measure of velocity or energy.
There are a few ways you could do it, involving pendulums, or the paper penetration method in the link mentioned above, but again you have all sorts of inaccuracies (actually, the pendulum method is probably the most accurate method of all, even better than the light chrono's, but it requires specialist kit, some pretty accurate measurements and some fairly advanced maths (ok, it's very simple maths actually, but still requires a good understanding of the physics involved).
The best method is to use a chrono. It's the only realistic way to get a decent idea of the velocity a BB is fired at. I have the shooting chrony which I use for my RS rifles. I keep meaning to get it out and test it on my BB guns, but it should be accurate for those.
The simple answer is, there's no real way for you to measure the velocity of your BB gun without a chronograph. Ask around on here, there is almost certainly someone near you who has one, or has access to one, that you can use. You can buy a muzzle mounted chrono for around the £50 mark (look up Combro) but they tend to be rare as hens teeth for long periods of time.
If you're anywhere near Swindon/Newbury/Reading I can run your gun over my chrono, just drop me a PM.