is there any magical formula or good rule of thumb you guys use to figure out how much live sand to add compared to dead aragonite sand? do i even need live sand or can i just get a detrivore kit?
It's my opinion that you can simply add the "dead" sand to a tank containing good quality, cured live rock, seed it with a bit of raw shrimp and let her go. It will become seeded in relatively short order (remember, everything is relative! Time needed for such things with salt tanks is going to be longer than fresh), at which point you will also have quite conveniently initiated the nitrification cycle and you can add your detrivores. I don't think you really need to use live sand unless you're in a real hurry to get things going.
One benefit of live sand is the diversification of life, assuming the live sand is healthy when you get it. You don't need it, per se, and there's no golden rule of thumb for how much to use (or even to use it), but it doesn't hurt to seed the tank with high quality live sand if you can find it. If you absolutely must have a percentage, shoot for 10% (completely arbitrary number )
Otherwise, stick with good ol aragonite sand, seeded by your live rock and maybe a good detrivore kit.