How to fix rounding issue involving placement system?

Hello,

I am making a building game for fun. Placing blocks on the ground works perfectly, but placing blocks on the side of other blocks has a problem.

Placing works normally in the positive X and Z direction but not in the negative X and Z direction. This results in the block being placed going inside the block instead of on the side (visual representation is below). There is no issue with placement in the Y direction. I know it's a problem with the rounding but I can't quite figure it out.

Positive X and Z: https://i.gyazo.com/49d548f5ae0479192d6b822cd0c32df3.gif?_ga=2.209657489.2076798991.1542575956-1841802524.1538965118

Negative X and Z: https://i.gyazo.com/7e1218e29642a10ffde2793077cd4d7a.gif?_ga=2.239001219.2076798991.1542575956-1841802524.1538965118

This is the section of code that rounds the X value so it aligns with my placemend grid. The same code is used to round the Z value:

local factor = 4; --factor aligns position with placement grid
position = mouse.Hit.p; --raw position of block

local X = position.X; --raw X position
local tep = X/factor; --raw X position aligned with grid
local d, tepd = 0;
tepd, d = math.modf(tep);
if d >= 0.5 then --rounds X accordingly
    X = (math.ceil(tep))*factor;
else
    X = (math.floor(tep))*factor;
end

But wait! There's more!

After some experimenting I found that if you switch the math.ceil() and math.floor() functions on lines nine and eleven you can place blocks on the side in the negative X and Z directions but not in the positive X and Z directions. The code after the switch looks like this:

--all of this is unchanged
if d >= 0.5 then --rounds X accordingly
    X = (math.floor(tep))*factor; --switched
else
    X = (math.ceil(tep))*factor;
end

I'm assuming there is a way to have it set up such that it works in all directions along the X axis and Z axis. But am I going about it the wrong way?

Any feedback is appreciated!