125ml in 5 ltrs of petrol would be 50:1, and not 40:1, which requires a bit (a teeny-weenie bit more oil)
However, the difference is so minute, I wouldn't worry about it.
Black oil from the exhaust is normal in a two stoke, Igf you are finding it's running well enough, it's best not to start twiddling around with mixture screws, which, if you don't get it right, will result in a seized engine from making the mixture to lean.
leave it well alone if it's going allright!
As an aside, I allways run my two stroke equipment with too much oil. Far better that, than not enough, and I like to dissapear in a blue haze...... it keeps the insects away..
