Trimming a boat to its optimum angle depends on a variety of factors. It also requires an intimate knowledge of your boat. How does she respond when full of water? Is there a day fuel tank forward that will affect trim, or does she carry all the weight in the middle?
The more you run a boat, the more you'll understand her nuances. For example, I used to run a 38-foot express cruiser that would usually only need to be trimmed when we were full of fuel. All 300 gallons of diesel were under the cockpit, and as we burned fuel she began to run at a better angle. Three hundred gallons of diesel weighs about 2,100 pounds. In addition, it is key to understand what is happening below the waterline.
For the sake of clarity, let's assume you're operating a boat with inboard power, either straight or V-drives. Shafts, struts and props come out of the hull at an angle, although some more than others. Therefore the props will push water down, pushing the boat up at that same angle, either at the engine or V-drive location. Thus trim tabs are necessary on planing and some semi-displacement hulls. A good hull design will incorporate lift characteristics, which will determine how much tab is needed.
To then properly adjust your tabs, put your GPS on the "speed over ground" display. I like to adjust the tabs after reaching full cruise rpm by pushing the switches for two to three seconds and waiting 20 or 30 seconds for the speed to increase or decrease. Again, as you cruise your boat you'll begin to understand how much adjustment she needs. But be sure to monitor boat speed and rpm.
It is also important to observe the sea state. In most cases, I prefer no tab in a following sea, since a neutral tab will keep the bow high. As the boat runs off a wave, neutral tab helps eliminate a bow steer reaction and prevents the bow from being pushed into the back of the next wave if the seas are stacked. In a big head sea, a neutral tab and high bow will help keep the decks drier and prevent the bow from diving like a submarine. Trim tabs function independently of each other. This can be useful for leveling, offsetting weight distribution and countering for beam seas.
Still having issues? This YouTube video gives some great troubleshooting tips.