I use our kitchen scale to weigh my vehicles.
ok thank you. do you think 3s will be plenty or should i do 4s just to be safe
3s with 6x4.5 props should fly fine. 4s would be a real racing setup and that is not what Navio is intended for, I would say.
You also have to be careful not to overload your ESCs and motors. You did only write simonk esc, but not the current rating. If they are 12A ESCs, 4s with 5x3 props might be possible. But you have to test everything yourself, all I write here is just based on my own experience and can vary from copter to copter.
when building a new copter, this calculator is really handy: copter calculator
it is not 100% accurate of course but should give you a good idea of being able to take off or not.