My Supra idles at about 600 rpm smoothly..
Look at the threads here.. tons of info...
http://www.toyota-supra.info/forums/supra-faq/
I personally wouldnt touch a car without the proper specs/information.. dont guess with a Supra.. do it right. Here is the TSRM
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/