That means 250 watt actual out of 600 watt advertised. You need to calculate your watts per foot off actual. If you’re light was a true 600 watt hid replacement, then saying you have 90 watts per foot of hid equivalent would work. However most aren’t anywhere close. You need to compare part levels to say one is a true replacement. The efficiency boost in higher end leds has to be considered when talking replacement watts. An Uber efficient 300 watt led can produce similar par levels to a 600 watt hid. But you’re not getting a fixture like that cheap either, so it’s all pretty relative to getting what you pay for. There are a few good values out there, but it usually comes down to cost vs performance. If you see one 600 watt led for $800 and another for $150, there’s a pretty good chance that a $650 difference in performance exists too.
Ok so my 600 w marshydro with tru draw 280 should be done at 600 correct?
Added to a 300w hps in a 32"-32" inch tent
That’s 900 watts.
I thought I was good at 580 wow 
I might be in overkill land .
It’s morning and I am only on my second cup of coffee so please excuse me if I misunderstood what you wrote.
The way I understand it your Mars light has 280w actual draw. That, combined with your 300w HPS is a total of 580w. Divide that by your square footage (6.25 sq/ft) and you have 92w per sq/ft.
However, the “watts per square foot” number isn’t the whole picture - PAR foot print for your light is (I think, doing this from memory) 680 at 24" and drops off to between 150-320 at 2’ from center.
Combining the two different types of lights adds complexity to this that I am not smart enough or experienced enough to comment on - but from an LED perspective I think for flower you could actually use two of those MARS 600w lights in a 2.5’ x 2.5’ growing area based on PAR. I’ll let @dbrn32 correct me. 
@Wishingilivedina420state Your 600w with true draw (280w) puts you at just under 45w per square foot true draw, which based on what I have read is very good for LEDs. Add in your 300w HPS and you are at 92.8w a square foot, which to be effective means you must be supplying a lot of CO2 and Nutes. Adding a 600w LED and taking out the HPS puts you in the same range, 89.6w per square feet.
PAR is also a key measure because it measures useful light at a specific distance. LEDs are supposed to have really good penetration, as do HPS, but all light falls off quickly with distance. Thus the challenge of growing large, tall plants. It is safe to say the taller you grow your plants, the more light you need. That is why you see so many SCROG and SOG grows. So, this needs to be factored into your equation.
As Bogleg pointed out each light has a footprint, which is really about how wide or narrow the light is directed and how effective the light is as it gets wider. This is also a key factor as they do differ with different lights, but in a 2.5" x 2.5" and the two lights you have I can’t imagine foot print would be nearly the issue as height. And at 90w true draw a square foot light is not likely an issue either.
Lastly, Dbrn32 is also right that different lights perform differently (that is what got me to start this thread). However, if you have a decent light it will perform at least on average with others. So, given the numbers used for these calculations are “average” numbers, I would be confident that your numbers give you a pretty good idea of where you sit.
DISCLAIMER: I am a researcher/info geek, but do not have a lot of experience actually growing.
If you’re targeting watts per foot for led, use the actual power draw. In this case, would be the 280 watts. Same for the hid, 300 watts.
Like the others are suggesting, par levels are a much better target. It will take the efficiency of your lights into account. If you have 300 watts of lighting running 1.5 ųmol per joule, 300 watts of a light running 2 ųmol per joule would be a 25% increase in light that costs exactly the same amount in terms of energy to run.
The only comparison I know of for hid is a de gavita, it’s around 1.7 ųmol per joule. It’s also considered to be “the light” for large scale hid growing. When compared to something like the hlg 550, that runs at 2.4 ųmol per joule, that’s how you get hid equivalent using less watts than hid. You’re not automatically equivalent because you have an led light. Especially if you’re buying budget lights on amazon. Most of them use inexpensive diodes that aren’t nearly as efficient as some of the more modern high end diodes and chips. On top of that, the individual diodes have a polycarbonate cover the light had to pass through, and then there’s a layer of glass or plastic for the fixture as well. Both will hamper the amount of photons provided.
The good news is that most are providing par maps. Look through the pictures and descriptions, they’re usually there. For veg, around 400 ųmol will do the trick. During flower, you want to be in the 600-1000 range. Less will usually leave your buds small and/or fluffy. More than that will essentially be a complete waste without supplementing co2.
As led technology continues to develop, compensating for efficiency will become more and more important. Shifting from watts per foot to ppfd will become more and more prevalent.
Nice post. I look then par level and do my comparsions. Thanx.