You may plan to have a certain barrel OD, your choice. Or you may plan to have a certain cap OD, your choice. They are more or less related, because you may want a step between barrel and cap, or not. Something between 0 and 2mm is nice, more would be less attractive to the eye.
Besides, you don't want too big a step between the barrel OD and the threads I suppose. I would say 0 can be OK, 1mm is an easy way, 2mm are more or less acceptable, more is less attractive to the eye.
The cap ID is important too, because it has to be large enough for the section. The section itself cannot be bigger than the thread. Besides, you need a big cap ID if you use a big nib, like #8, and will be related with the cap OD and the threads diameter. And a cap ID means a minimum cap OD.
Then, choosing a 12 or 13mm thread is a matter of playing with all these interactions, and with your own preference. You have to start somewhere : do you want a certain section OD, a certain barrel OD, a certain cap OD or ID ?
