Simon, your question requested ... "simple" and "basic rules". Everyone has provided that... and even more in depth data.

Of course small changes effect performance... look at what the outhaul and downhaul do... they make small adjustments to the sail the improve or depower your sails.

Since every boat, sail, crew and wind condition is different... you need to apply these general rules of thumb and test the thousands of variables to your conditions... and then try to remember what effect they had.