The better the beer and lemonade the better the shandy. One tip put the beer in first or it will "fob" and give you a big frothy head and overflow the glass or bucket.
If I ordered a shandy at the pub and they used lager not ale then there would be a complaint. If one wishes to substitute one of the traditional ingredients the buyer should specify the change. It's usually expected to be mixed 50:50 but like many things, you can do it "to taste".