I will answer your questions, but, I highly recommend you read the "Brians Users Guide" in the member docs section on this forum, it will answer many of your questions.
Summer is this, you analog out is on Cfg #2, and if i under stand you correcly your saying out analog output is #3 on his board, which equates to local var #2, in the modbus (since the anlog local vars is 0-3 for physical outputs 1-4 of outputs).
so your output termination in serial modbus will be "Modbus Serial plugin, Cfg #2, var #2"
this assumes your modbus analog outputs is on cfg#2 and your using the 3rd of the physical (screw terminal), on the analog out put row.
the resolution of his analogs is 256 (8 bit).
so the Brian line/rung for spindle analog output reads like this:
Spin DRO 99 -> formula: A*255 -> Plugin serial modbus, output, cfg 2, var 2
scott