Perhaps JayPC's game doesn't allow diagonal movement, so two diagonally connected floor tiles need to be connected horizontally or vertically?

In that case, using NON's example, reducing the increment of y to .5 would work, except for (nearly) 45 degree slopes.

To guarantee orthogonal connections, you could just generate a map with diagonal connections and run the result through a cellular automaton like this:

`turn .# into .. or .#`

#. #. ..

Edit:

*Or*, you're calculating slope incorrectly. 17/26 = 0.65 after all, not 1.53. To guarantee connected floor tiles, you don't need a lowest common denominator, you need the slope to be <= 1.0.