180 - 240 is a really big range. My first suggestion would be to get a more accurate test kit. 180 is fine, 240 is unacceptable. If your kit can't tell if your level is fine or unacceptable then it's not that useful.
In general if you're wanting to lower ALK just stop dosing the alkalinity additive and it will drop naturally fairly quickly.
The best water change would probably be something like 50%. For most people that's impossible. What I recommend is a schedule of water changes that's easy to remember and easy to perform. That way you'll be more likely to do them. 20% every other week is probably about right for you.