How much lemon juice should I use in a fruit loaf?

I have a recipe for fruit loaf that calls for the grated rind and juice of a lemon, I'm hoping to substitute this with some shop bought lemon juice, but I'm not sure how much to use. The dough looks as though it would be quite heavy, 350g plain flour, 2 eggs, 175g sugar, 175g butter, 225g fruit, 1tbsp treacle, 2tbsp milk and half a tsp of bicarbonate of soda. This small amount of bicarb dissolved in the milk is the only raising agent, so I'm thinking that the lemon juice is there for more than flavour. I'm also wondering if I should just use more bicarb?