Thanks for your reply. I think your post is pretty much on the mark.
Pretty new to asymmetric sailing so it is all trial and error at them moment. Thanks for the maths!
I got myself a copy of Asymmetric Sailing – it suggests loosening the gennaker halyard and then attempt to swing the gennaker more to windward so it acts more conventionally. I’ll give it a try. As a lake sailor we get plenty of light wind days to try these things out! The modest sailplan works against us in the light wind.