Yes, that ought to give you the same timing accuracy as your PC clock. Depending on your PC clock accuracy, you may need to update it from the web more frequently than Windows does by default.
I suggest you program individual timers for each On/Off event. (A delayed macro will be timed from the _end_ of the previous event rather than from the beginning, and an X10 On or Off signal takes about 1.2 seconds to be transmitted.)
(One of the main problems with running from a downloaded schedule is that the CM15A clock is maintained in sync with the AC line frequency. Barring a power outage, the power company attempts to keep this correct on the average, but it can swing back and forth a number of seconds during the course of a day or even an hour.)
Let us know how this works for you.