There are lots of opinions on where the toilet paper holder should be. Some people get quite aggravated if the holder is not where they expect. In a household of multiple ages, heights and weights, the placement can be problematic.
The Standard Approach
Option 1 is to place the holder on the wall to right or left of the toilet, in the standard position of 26 inches off the floor and 8 inches forward of the front of the toilet. This is great for most adults and children, as they can reach it easily from the seated position. Measure straight up from the floor and mark the screw locations using a level so that it will sit straight.
The Back Wall
Option 2 is to install the toilet paper holder 26 inches above the floor, but on the wall in back of the toilet, about 8 inches above the toilet tank. This is a good position if you want to keep the wall free or if there is not much room between wall and toilet.
Option 3 is to place the paper holder 26 inches above the floor but straight in front of the toilet (assuming that wall is reachable from the seated position). This is advantageous for infirm people who do not want to have to turn to the side to reach the toilet paper. If only one person will use the holder, keep it slightly to right of centre if the person is right-handed or just to left of centre if he is left-handed.
Option 4 is to try the blind method. Take a pad of sticky notes and have the members of your household write their names on notes. Close the lid of the toilet and have each person sit down and reach with eyes closed for the spot he or she expects the toilet paper to be. Have each person place a sticky at that spot. Then place the holder at the midpoint of everyone's reach.
Option 5: Forget the whole idea of mounting it on the wall and get a floor stand that holds about 8 rolls. You can place it wherever you like and you will almost never run out of paper.