Paul, Sad man that I am.
I've been thinking about the tape we were using. as it starts at 10cm we would have to reduce the measurements shown by the tape, not add another 10cm (effectively the error is 20cm).
So the first measurement (longest side to the left looking from the tower) is OK, because I took 10cm off. All the others along the back the far right triangle are all 20cm too long. Hope this makes sense and that I caught you before you started drawing?