Yes I was refering to cleaning the gravel. You say most say 10%... That's actually all I do. The key is to do it often enough to where you will not need to do a big change... Balancing out the waste/live bio ratio is the most important for the "quality" of your water. Lol as far as parameters... I don't worry about that as long as my buddies meet me at the glass for feeding 3 times a day.
A couple things:
1. I'm a huge fan of water changes. IMO they are better for the overall health/color of a fish than what foods you feed. I do two 85% WCs on a 90gal holding a solo 5in trimac.
2. I can't tell from the pic, but is your heater positioned vertically or horizontally? If you turn it horizontally, you can drain the water farther down. It is also more efficient b/c it increases the surface area of the amount of water being heated.
40% is alright. If I had your tank, I'd personally do 50% and gravel vac really well just because you can never really get the gravel to be 100% debris free. Then again, I'm a big fan of doing daily water changes(I have bare bottom tanks only). 15% everyday on my tanks except 50% on saturday.
To figure out how much water to change, you need to take a nitrate reading. Most aim for 20-40 ppm at the time of the water change. A 50% water change will drop nitrates by 50% assuming your tap has a 0 reading. If you have over 40 ppm when you do a water change, you need to do water changes more frequently or change more water at a time. The more water you change, the more nitrates you will remove. The more nitrates you remove, the better the color and growth of your fish will be. Some say that a fish's system associates low nitrates with a large, open body of water therefor stimulating growth. Whether this is true or not, I don't know. But, clean water is the key to a healthy fish.