Not exactly, it also depends on whether or not it is a balanced amp and or if the amp has an internal DC/DC converter (as a lot of high power car amps do)
When no DC/DC converter is present and not balanced use the following formula.
Power supply voltage (if dual supply then the +/- voltage) and divide multiply that voltage by 0.35.
When no DC/DC converter is present and the output is balanced use the following formula.
Power supply voltage (if dual supply then the +/- voltage) and divide multiply that voltage by 0.7.
When a DC/DC converter is present all bets are off.