Yup the great science of aperature. Hard to fathom for some. difficult for the rest. The general rule of thumb is this 1/3 front -2/3 behind the subject. Now the tricky part is this gets "deeper" with the bigger the number on the aperture ring. On older lenses there are numbers on either side of the focus line on top of the lense. these numbers show corresponding to the "depth of field" on the lense. On newer lenses these numbers on top of the lense don't exist anymore so you kind of have to just learn what they are. Also on most camers there is a depth of field preview button usually located on the lower front of the body the you would activate with your right hand finger.
so bigger the f stop the bigger the depth of field. And with thirds rule you can cheat this distance by putting your subject in a different place than what the camera wants to focus on. Best practice is on a long fence.
this is probably the hardest concept in Photography. Read more from the Art Wolfe book that you have from me. It should help.