The biggest problem with "Pre-Designing" a pen , either by hand or software , is that you cant really tell whether a complex design will work when your square blank is turned round . To keep a pattern consistent as it is turned you have to think in three dimensions . What looks good in 2D changes , sometimes allot , in the third dimension unless you make some very complex angled angle cuts to keep the design consistent as it gets smaller through turning and changes due to the square to round changes .
When people like Jeff Powell make pens that have designs or patterns in them they have to distort the design or pattern so that it looks correct as the blank is turned round . The design or pattern changes in width more then in length as you turn it round and it gets smaller .