Most folks probably already know about the Discount Tire Tire Size Calculator as a great tool for comparing different tire sizes and the impact of different tires on the speedometer reading - https://www.discounttire.com/learn/tire-size-calculator.
WillTheyFit is another great site that generates a visual picture of different tire and wheel sizes, including Offset, but its speedometer calculations aren't very accurate - http://www.willtheyfit.com.
I just googled converting backspace to offset and came across a couple of useful sites.
Wheel Specialists offers a chart to convert backspace to offset - http://www.wheelspecialists.com/wheel-specs. The only drawback to the chart is it is the backspace values are incremented by 1/4".
GTSparkplugs offers a bunch of different automotive calculators covering gas mileage/costs to electrical, carburetors, tires & wheels and more - http://www.gtsparkplugs.com/AutomotiveCalculators.html. I found their Backspace to Offset calculator very useful since the Slingshot specs focus on wheel offset but lots of other sites use backspacing instead.
If anyone has other favorite calculators for auto-related issues, add them below so others can try them, too.