Not sure if this is the best method but I would take a long piece of paper or backer and first make two marks indicating how long the lettering will be and then mark a flat baseline. Wrap that paper around / over the curved item that you want to letter (car windshield, bucket etc.) Put the center of the baseline at the height you would like it, then measure from some common point that height (top of bucket or?). Now measure down from that common point and mark the same height where the beginning and end mark for the text are. Remove paper and lay flat. You should now be able to measure your height above or below your baseline at your start and end points, with that you can make a curved baseline with the pen tool in illustrator or corel to conform your new text to. The text will come out like a smile or inverted smile if you will but should appear straight on your substrate. I have probably overcomplicated this.
