Railings leave much to be desired in terms of truly flexible design. When you need to create a railing that requires different baluster panel sizes, in particular at its ends, things start getting fussy.
This is an example of a railing that has been "designed" to use two different panel sizes. The overall length of the railing must be factored in at the outset. Railings are just like wallpaper which has a defined "repeating" pattern that must be plotted/planned out before beginning work.
Assuming that the pattern requires a repeating 24" typical panel interupted now and then with an 18" panel for a design esthetic as well as finishing up with 18" panels at each end because the overall length "required" is 20'-0". These are nice clean numbers obviously and if the real world defined some messy dimensions, the first and last panel's size(s)would need to be adjusted to match those messy numbers.
This is the final result, a 20'-0" railing with 24" and 18" panels.
This is the Baluster dialog captured in two stages because it wasn't possible to capture a scrolling window for some reason with SnagIT.
This is the rest of the dialog.
This is the sketch length, one long segment @ 20'-0" long.
The key parts of this pattern:
The first panel uses a 9" offset which is half the panel width
The distance from center to center between 18" and 24" panels is 1'-9"
When laying this sort of thing out you need to start with the overall required length. Define how many "normal" panels fit and what is left over. Then decide whether you are splitting this left over amount between start and end or placing it entirely at one end. Once you've figured this out you can start defining the railing baluster pattern.
I've posted the example project file that these images are based on HERE.
An awesome improvement to railings would be to permit a baluster to flex like a curtain panel in a curtain wall. Ironic a bit because if I recall correctly the very early railing tool was the curtain wall tool or vice versa?