You have to decide how even you want the amounts withheld to be and how many times you want to have to make an adjustment.
In the past when the system my company used only allowed you to state a percentage, I found I was able to hit the target reasonably well by setting the percentage 3 times. The key was to keep track during the year.
The first adjustment would be done to guess a percentage for the first paycheck of the year. Then in the spring my company give salary increases. Once that was received I used a few lines in a spreadsheet to determine how many checks at x% and how many checks at y% would be need to reach my goal. There was a limiting factor to make sure that last check of the year still had enough withheld that I received the full match.
That last point for me was very important. If I hit the annual limit in October, that would seem to be good because I then had extra large checks late in the year. But that also meant that those last checks of the year didn't trigger matching funds so I could be leaving hundreds of dollars on the table.
Using the three settings I was always able to hit the annual limit on the last paycheck, and still get all the match.