The shade you can figure out from the extent of sun angles during your study and the extent will also depend on height of shade.  If you are too busy to calculate, just take the cloth to the ground and encapsulate the plot.  The shade cloth may capture and evaporate a small amount of rain, or increase some rainfall splash out of plot, bit probably minimal.  If it rains the adjacent ground next to the rain shelters will be wetter, some is apt to get into soil and affect the water to the plants.  The microtopography would have to such that the rain shed by shelters would be removed, and not flow back into study, as well as no surface flow entry from outside.  It may be easier to irrigate, than to prevent access to water.  It has been a long time since my plot study days, but I seem to remember 1/2chain (33 ft) or 10 m as a good plot separation minimum.  I dont know where that came from, so maybe you can get better advice.  Make sure the soils, topographic position are the same unless varying is also part of study.  

As you have well commented, the surplus effect depends on the height of onstáculo, but can also affect other factors such as the slope and direction and wind speed. So it is not easy to establish a limit separacón or minimum plot size. Ten meters away'll look adequate for most cases, if you do not have time to make a more accurate calculation.