The rim widths between alloy and steel should be appoximately the same, providing you are sticking with the same rim width i.e. WM-2.
The offset for a Commando front disc wheel is 1/2" from the rotor mounting surface with a WM-2 rim, Commando rear disc is 3/4" from the rotor mounting surface with a WM-2 rim, and a Commando rear drum runs right down the middle, equal measurements both sides from the hub spoke flanges.
If a wider rim is to be used, reduce the offset by half of the difference in rim widths i.e. if switching to a WM-3 from a WM-2, reduce the offset by ~1/8".
HTH,
Derek