Are you sure you're using 6 cylinder online calculators? I'm basing it off the fact that with a 6 cylinder generally 1cc of injector flow = 1whp of capability. To run the numbers you have to go backwards from horsepower.

According to Ray Hall (If I remember correctly), hp = 10.87*lb/min of airflow. Assuming wheel horsepower is approximately 15% less than crank horsepower (maybe more in a Porsche) If you're making 590whp, that would be around 695 engine horsepower (using 15% loss) - with these numbers that is around 64lb/min of air.

I don't know what lambda/AFRs you all generally tune to, but given a lambda of .80 (11.76AFR) that would mean that your engine is consuming 5.44lb/min (326.53lb/hr) of fuel - divide that number by six and you need 54.4lb/hr through each of your injectors yeilding a 91 percent duty cycle...approximately of course.

On an 8 cylinder if you require 326.53lb/hr - that would mean you only need 40.82lb/hr out of each injector putting the duty cycle at approximately 68%.