Stepper Motor Power Question

OK I'm pretty much a newbie so far as building is concerned. I'm putting together my first gantry router loosely based on http://www.instructables.com/id/DIY-CNC-Machine-36x22/ for use on wood and Sikablock type products. Being a tight-wad I have chosen to use stainless steel all-thread drive screws with double anti-backlash nuts made from Delrin using my vintage Myford S7. Having a Halda gram gauge I'm thinking of being a bit scientific by using it to determine the force at a given radius (and hence the torque) required to start the lead-screws from rest. From this I'm hoping to determine the minimum torque I need from the stepper motors which I have yet to select. What I'd like to ask the panel is "What sort of allowance over that which the Halda tells me would you recommend?".