The single best way to figure out how much torque you need to hold a rotating arm is to buy yourself a
luggage scale or a fishing scale or something.
Disconnect the arm from any gearboxes or motors so friction won't throw off your readings. Tare the luggage scale, put the rotating arm horizontal to the floor, hook the scale onto the arm somewhere and measure the weight. Multiply the weight by the distance from the pivot to the hook and you've got a fairly accurate measurement of the torque you need.
Note that you want your transmission to stall at something like 4 times this torque or better to account for losses, actually moving the thing, and not blowing breakers. JVN's calculator can help you more with this aspect of things.