Tim,

the datum line is often the thrust line although in the case of the Curare I don't recall whether these coincide. In any case, the plans will have a datum (a common reference point) about which you can measure offsets. This might be the top of the fuse before the curved portion of the decks begins. The datum is meant to be a horizontal reference off which you can measure things like incidence and engine thrust angles.

As you say, transfer it to the fuse sides and use a spirit level to make sure the datum is horizontal. You then measure offset of the engine with respect to this level point.

The datum in usually of relevance in the side view since the datum in the top view is the fuse centerline. If you think of two sections (planes) going through these datums (one horizontal and the other vertical), they will intersect. I seem to recall that the Curare has this intersection at the thrust line (that is, the engine crankshaft at 0-0 sits on this line prior to introducing offsets to compensate for torque).

If you need to extend the datum along the fuse side because it curves in at the nose, use a light straight edge to extend it.

If your FW us at 90 degrees to the datums, then you probably only need a washer or two for the offset angles. If you have only one washer at the moment, your FW is likely not vertical - the downthrust looks too extreme.

David.