[Probably most will have already figured this out but some may have not.]
For those of us struggling in the feet&inch side of the world, there is actually some elementary built-in ArchiCAD coordinate input math we can use to make additions almost as simple as metric: say you need to add 2'-11' to 1'-5 1/2", all the math you need to do is (2x12=24 + 11 = 35 in, which is what you will add to your 1'-5 1/2'") so you go to the coordinate field and type 1-40.5. ArchiCAD will figure out that 1'-40" is actually 4'-whatever and display the correct notation.
Same thing is true for inch fractions, which you can use not only to get 1 1/16 from 17/16, but also actually to get the halfs, thirds, quarters, sevenths, whatevereths, of any inch dimension --131/2 will give you your 65.5 in, 18/9 will give you 2 in, etc.
