Neil,
Not too much in the way of suitable books which go into the nitty gritty .... perhaps I might have to write one sometime ...
(a) datum.
If you play with various datum positions, you will see that the shape of the envelope varies from
(i) the typical GAMA shape (thin thing sloping from low weight/forward CG to high weight/aft CG) - forward datum
(ii) upright - which you can then expand to make the desired boxy shape - mid range datum (best generally will be somewhere in the back half of the envelope - for lighties, generally the most aft limit works well)
(iii) similar to (i) but sloping the other way - aft datum.
Don't waste time .. go for the (ii) option to get the best results and fine tune it from there.
For the calc side of things, with a "standard" datum, the formula is something like
IU = (mass x arm)/convenient non-dimensionalising number
If you have changed the datum and still need to refer to, say, the OEM POH (and this is the general case) then you need to "change" the arms to reflect the new datum while retaining the link to the OEM information.
If you draw a piccy, you will find that the answer becomes something like
IU = (mass x (OEM arm-datum))/number
where the new datum is defined with regard to the OEM longitudinal station diagram.
Think about it - if the standard arm is, say, 100 inches then the arm is 100 from the OEM datum. If you now decide to make the datum, say, 40 inches (referenced to the OEM FS), then the "new" arm is 60 (ie the distance from the new datum to the location). This can be summarised as
new arm = OEM arm - new datum = 100 - 40 = 60
Nothing stupid about it ... just takes a little while to get used to thinking a new way ..
(b) Sorry about the acronyms ..
TCDS
Type Certificate Data Sheet. These are an extension of the Type Certificate and, with the AFM, give you all the limiting data relating to the aircraft Type.
IU = index unit = moment/convenient number
FS = fuselage station, ie the loading arm picture in the POH
(c) re your sheet ...
The mass figures at each line define the standard (mass) load value for each division shown on that particular line.
The sloping lines are a graphical trick to improve the accuracy of the sheet's execution by removing one of the interpolations at each line. Each sloping line to the next as a distance represents the IU (or moment, if you prefer) difference (delta) relating to adding (or subtracting, if you go the other way) the standard load (ie the mass figure described above) at that station.
Suggest that you keep asking the questions here as others will be interested in the answers... and if my answers don't get the message across, ask again and I will rewrite them until I get it right in terms of your understanding what I am trying to get across.