Hi Phil and wecome to the forums,
I see rabbut and others have been taking good care of you. Even though the tank has racked up quite a few weeks with fish in its not quite clear to me that you are not still right in the middle of a Fish-In cycle.. I think you still likely are.
Your questions about how often or how much water to change made me think the following might be helpful: Its like a detective game on your part. When you're in a fish-in situation you want to be pretty regular about testing for ammonia and nitrite(NO2) twice a day. Most people do morning and evening when they are home. You're expecting that between testing times the fish (and excess food and other debris) will have possibly built up more ammonia and nitrite(NO2) than your baby bacterial colonies in the filter can remove. So what you're hoping is that neither toxin has built up higher than 0.25ppm. If it is higher then you need to adjust your water change size to be larger or to be done more frequently. If you're closer down to zero, then you know you're doing ok or can ease off of the frequency or percetage some. But you need to keep on testing and adjusting. Once your filter has kept the ammonia and nitrite completely at zero readings for two full days without you doing any water changes then you know you're probably cycled and then you just keep up the same frequency of tests for the next week but expect not to have to do water changes, say until the weekend. After that you can ease back on the frequency of testing and begin to carry out the good habit of a weekend 30-50% water change with gravel clean, which is what will maintain a good tank after its cycled.
Hope this helps some in the sense that its always good to hear it different ways from different members,
~~waterdrop~~