How much rum to cola would I need pour into a 250ml glass so that I have a drink with 6% vol alc content?

I wouldn't know how to figure it out mathematically. When I make a drink in a tall glass, let's say 16 oz glass, I measure out 1 shot glass of alcohol and it is fairly low as far as actual alcohol content. If I want a stronger drink, I will put 2 shots.

Of course, I am not in the UK, so you might be doing things different over there, lol! Have a nice day.

I don't know, but I'll tell you how they mix a drink in Puerto Rico. I was there once and asked for rum and Coke. The lady poured a small amount of Coke in a glass and handed me the rum bottle to get whatever amount I wanted.

You have the most awesome chemistry teacher EVER.