Using 1.5 inches of padding seems perhaps a bit excessive, but the resulting flange is useful at the assembly stage for attaching C-clamps. For the most part, though, having an extra flange does little or no harm and is potentially useful. Remember that if you shorten your padding, then you may also shorten the bending length and will have to recalculate the bending length modifier. These are the cut lengths. Cutting I cut the lengths from the conduit using a high-tension hack saw.
Some people recommend using a chop saw.
The length of the flat part should be marginally longer than twice the distance from the vertex hole to the pole end, in order to allow for the poles to overlap without their butts hitting other poles. For the 100.5-inch diameter dome, using the lengths described above, flatten each end at least 2 inches. The flat parts at each end should be parallel to each other. If the flattened pole ends are not sufficiently parallel, assembling the dome will be much more difficult. Measured the distance carefully using the distance between vertex holes calculated above.