If you don't have a gauge, aim for something just above 1/2 and you'll be pretty good. There is a range that's pretty good, not just a single point. If you want to really fine-tune it you need a gauge. A vacuum gauge would help, too. 75% load roughly matches with 12 psi MAP (where 14.7 is ambient or engine-off).
Many bsfc charts are centered at lower rpm than the one shown above. Try this:
http://ecomodder.com/forum/showthrea...-got-1466.html
Again, my personal experience is that low rpm has its own advantage so you're better off keeping it on the lower side of what the charts show.
That being said, you're driving a Metro. That's a tiny engine. It may be that it does better at higher rpm than my 1.6.