Quite an in depth subject and much depends on the boat, the motors, mounting height, sea state, are you running for comfort, speed, safety, launching etc. I would recommend asking a competent examiner to accompany you on your boat and take her through a series of trials at different settings and explain the results to you. Much of this should have been covered in your skippers course? The advise you have been given is quite common but generally wrong as it negates hull dynamics and hydro dynamics and assumes a flat sea. I would generally start from the opposite end - motors trimmed fully in. Set your cruising rpm and measure your speed. Start adjusting the trims and watch the effect on your speed. Once you have found the maximum speed at a given rpm (probably about mid position on the trims but dependant on all of the above factors), you can then experiment with different rpms. You ideally should have at least 4 positions for the trims memorised, lowest planning speed, most economical cruising speed, flat out position, and then the difficult one - the most comfortable, which will tend to be somewhere in the middle and is very dependant on sea state. You can then add launching and beaching positions, head and following sea positions and also lateral trim positions where you will want to lift the windward side slightly. In saying that however, a good rig should require minimal adjustment except in extreme circumstances. The main thing to remember with a following sea is to run at least slightly faster than the sea and trim the bow up so that you do not dig a sponson onto the trough. Do not trim too far though or you will create more drag at the stern than you are trying to lift.