A few years go, the brilliant @davescm posted the following formula and the math he used to create it.You may want to apply it in order to determine the minimum ppi recommended based on viewing distance.
The ppi required can be calculated directly from the viewing distance.*
ppi required = 6878/Viewing distance where viewing distance is in inches.
So for your example5 feet away -- ppi = 6878/(12x5) = 6878/60 = 115ppi
* The math that arrives at the formula above is shown below:
A good human eye can resolve 60 line pairs per degree i.e. pairs of black and white lines. So in half a degree we would have 60 single lines.
Simple trigonometry means that the width containing those 60 lines can be calculated by: Width of 60 lines = Viewing distance x tan(0.5°)
Once we know the width for 60 lines (or pixels) we can calculate how many lines per inch can be seen by using 60/Width of 60 lines.
So putting those together gives us ppi = 60/(Viewing distance x tan(0.5°)) = 60/(Viewing distance x 0.0087269) = 2/(Viewing distance x 0.000291)