My current rotation methodology is seven pens each in rotation for 14 days.
Reasons for this methodology:
One of things that sets me apart from the average fountain user is that I use a database to select my next pen, nib and ink (PNI) combination to come into rotation.
I am a Mac user at home so the relational database application I use is FileMaker Pro, of which I am still using version 16 (version 17 is available but due to cost and lack of enhancements that appeal to me I am holding out till version 18).
I am quite the archivist so I have backups of all my fountain pen database versions dating back to early 2011. Sometimes I open an older version just to look what variant of logic I was using then to choose my next PNI.
Ok down to brass tacks, my selection method is thus:
I rotate through three selection methods, top pen as starting point, top nib as starting point and lastly top ink as starting point.
What defines top pen, nib or ink? The algorithm for this is as straightforward as it has been in years.
For each pen, nib and ink I calculate a historic days between uses value for a maximum of the last six times in use. I also use a current days since last used value (DSLU) to give a maximum of seven values.
I then calculate the average and median of those up to seven values and derive the final single value as the highest of current DSLU, the average or the median.
The above image is the top section of my top pen calculation table. D0 is current DSLU, D1-D6 are the historic days between uses, AvD is the average of D0-D6, MnD is the median of D0-D6 and AD is the single final figure which I use as top pen. As you can see DSLU score dominates the score for the top eight pens with only the ninth pen having the median as its highest value.
Nibs tend to be more variable with what value dominates. The second item has a top score using average and so has the fourth, the fifth item where median dominates has only been out of use for 2 days and is a definite possibility of being used again soon.
Inks are exclusively dominated by DSLU. Because of this DSLU dominance I added a factor to the ink selection to counter this. I gave each ink a number between 1 and 7 to represent the days of the week with 1 being Sunday and 7 being Saturday. When I select an ink for rotation I only select from the pool of inks that have a day of week number that matches the day of selection.