If so, how much would be needed? A rough ratio of?
I'd agree with that about 1 scant teaspoon per mug o Plain Flour
1 heaped teaspoons Baking Powder per 2 mugs / 8oz Plain Flour
2 heaped teaspoons, or 4 exact teaspoons of (fresh) baking powder per 1lb plain flour
Don't be over doing it with your baking powder or you get that awful bad vegan cake taste
If you need extra lift try to be using things like live yoghurt, and emulsifiers like lemon rind/juice (as well as, not instead of)
If the baking powder has been hiding at the back o yer cupboard for a while test it with some vinegar and see how eagerly it froths up, then clean yer pans and hob with it!
By using plain flour and adding baking powder instead of Self Raising Flour it cuts down on storage space and means flour will be fresher. Even better