It doesn't quite work like that. Its not simply Vcc x Iout. You need to multiply the voltage drop, measured from the output to the rail its pulling towards, by the current through it. The voltage drop will increase as the die warms up, increasing the dissipation and further warming the die.
Its a lot of work to even roughly calculate the limits at your max intended operating temperature, and it may be simpler to set up a heated test jig to take some measurements of a few chips, derate by 50% then check you aren't too close to the package or individual output dissipation limits.