If all the datums are positives, draw the datum in the front.
Good idea also to draw the LeMac, MAC, and whatever else information they provide.
If you draw the datum at the nose, then it's linar scale, 0 at the nose and whatever measurement given.
All you need to figure out is where the MAC starts and ends, normally they give LeMac or C of G position, with this you should be able to calculate the different positions.
Example LeMac is 10m, MAC is 1m, C of G is 25% of MAC, your C of G would be at 10.25m.
Some questions however put the datum in the centre, then you need to use Plus and Minus signs, in front of datum Minus, behind datum Plus.
Hope this helps.