Looks like you've found your answer (and that calculator that @mrehorstdmd linked sure is handy, haven't seen that before!) but I'll add for others that one thing to take into consideration when looking at the power calculations is not just the W/cm^2, but the W/cm^3 as well.
W/cm^2 will give you a good idea of your bed's maximum temperature, but it won't really give you a good idea of how long it's going to take to heat up. I inititally chose roughly 0.35 W/cm^2 for my heat bed, but I went with a 3/8" (~9.5mm) thick aluminum tool plate which meant my heat up times were quite long (10+ minutes for PETG temps). I had based my initial calculations off of a Prusa i3 clone that had a ~3.5mm thick aluminum bed and a heater that provided 0.3 W/cm^2. On that machine, the heat up times were great, so I figured that just matching (or even exceeding a little bit) the W/cm^2 for the Hypercube Evo build that I'd get similar performace. Obviously if I had sat and thought about it for a little bit I would have realized that my heat up times would be roughly 3 times longs, since my bed was roughly three times as thick!
That's why I switched to a 700 watt AC heater mat to give me 0.7 W/cm^2, or 0.073 W/cm^3 (instead of the 0.036 W/cm^3 of the initial DC heater mat). Now I can hit PETG temps in just a couple of minutes!
As others have mentioned though, certainly make sure you do your due diligence if you go the route of an AC heat bed. I used a proper, high quality SSR from a reputable manufacturer, electrical fuses in-line with the machine, and a thermal fused bolted to the aluminum bed itself just in case the SSR fails. The machine is well grounded and the cables are strain relieved. Mains voltage is certainly nothing to play around with, especially when it's a machine that you're not going to be watching all of the time!
