Easy way to paint stripes: use ODD number of stripes so top and bottom are existing room color, make stripes in even inch (or mm) increments, and use the bottom stripe to soak up the odd bits of measurements.
Here's the real trick for perfectly crisp lines: seal the edges of the painters tape with some of the base wall color before painting on the contrasting color. Don't paint the whole width of the tape. Just a bit along the edge of the tape on the side where the stripe will be painted. Let that paint dry and then paint your stripe color. great idea!