First thing is I would say 1/8th stepping is probably best, next is it is best to calculate yourself rather than use Mach. Using Mach will be extremely accurate IF your measuring is accurate but without something like glass scales you are only going to get close.
So best thing is to work out what you have as a step per unit, set the accel and Vel low and start increasing the Vel until your motor stall (screaching sound) then back off a good bit and increase the Accell until the same happens. You then have to do a juggling act to find where your machine is happiest with regards accel and Velocity, you may prefer faster rapids with a slower accelleration or vice versa, personally I would go the slower rapids faster accel.
Hood