"bsfc changes with load and throttle"
Isn't that what the vertical axis is? Isn't half throttle the point halfway up the chart? That's how I've always read it. In the following chart, the best bsfc looks like 3,000 rpm (that high?!) and 55 Nm, or about 2/3 of max. Wouldn't that be (about) 2/3 throttle?