Here is another one I've been using http://www.1010tires.com/WheelOffsetCalculator.asp but the one you found is pretty neat
Just found this and thought it very helpful for what I'm planning to do
http://www.rims-n-tires.com/rt_specs.jsp
plug in whatever you now have and what you are thinking of going to and it shows you the changes in clearance
2009 MX-5 STR and track toy
2003 Z06 -- Not a Miata, but 3200# and 400 RWHP is fun
OTM -- 2012 Sierra, 2016 Impala (Appliances)
Here is another one I've been using http://www.1010tires.com/WheelOffsetCalculator.asp but the one you found is pretty neat
'06 RSX Type-S NBP