I still have a problem with this claim of yours. If a weather forecast says "There's a greater than 50% chance of rain today", how is that either true or false? Even after today, the actual occurrence of rain will have depended on location. Would you say that such a statement is false but still useful, before the fact?

I do agree with your position that sincerity is important in judging lies, but I have a more detailed explanation of lying: I claim that we make assertions based on a scale of beliefs that we hold, not on just truth or falsity, but a scale I can mark with words: certain, quite likely, possible, uncertain, unlikely, very unlikely, certainly not. A lie, then, is an assertion that (insincerely) mis-represents of one's degree of belief in that assertion.