sorry i know nothing about this subject. so for a 600w hps, its still 120 v right? thats exactly 5 amps. will it work or would it be too iffy?
You'll have to see what your strip is rated but if it has a 5 amp breaker and you are pulling 5 amps, the breaker will pop everytime you turn the light on. Therefore it will not work. 80% is a good rule of thumb as mentioned.
Also, why not just run straight to the outlet with no power strip ?
Outlets have 2 receptacles, run one straight to the light and use the second one for the power strip for the things that use less current.
Also are you in the united states or europe ? Europe is 230 volts, so for 600 watts you would be pulling 2.6 amps.
Yes you are using the formula correctly.
Tell us what is going on so we can get you a reliable solution, that is why I asked you the additional questions.