I change around 50% every week.
Darren, the fundamental point to remember is that so long as you roughly match the temperature, pH and hardness of the fresh water to that of the tank water, you can't change too much water.
A tank which has the water changed regularly will have similar parameters to the water from the tap anyway, so make sure you change some water at least once per week, and you only really need to concern yourself with matching the temperature and dechlorinating properly. Keep that in mind and water changes will always be beneficial regardless of whether it is 10% or 90%.
Follow Colin's rule to determine how much to change each week. Start with 30% like you are already doing and monitor the nitrate level. If the nitrate steadily increases despite your water changes, that is an indicator that you need to change more water. If changing 30% weekly holds the nitrate level relatively close to that from the tap, your water changes are most likely sufficient.
Follow Colin's routine that he describes above and you will be just fine.
Remember, you can't change too much so long as you bear in mind the points above, but it is quite easy to change too little.
Hope this helps.
BTT