So I would just raise the seatpost to the mark that says "this high and no higher" and measure from the bottom bracket to the top of the seat tube. That seems simple enough. I will also use Oakleaf's suggested method and see how they compare

Thanks for the tips - from the base measurement I took this morning it may actually turn out to be a 17 inch bike - which would explain why it is too big for me and I know someone who might be interested in it IF it is a 17 inch I need to make very certain though - I now wish I hadn't removed that sticker.