Formula for block-snapping in building systems?

Hello! I've been working on a simple building-system. I have a problem where when I move my cursor to the left (or right depending on which way I'm facing) it snaps halfway through the block instead of the mouse actually being outside of the part.

TLDR:

Join this game, Press Z, and move your mouse halfway through the block and it'll snap instead of the mouse actually being outside of the block.

https://www.roblox.com/games/1410833738/Building-Example

More information can be given via Discord - Goat#2976

local RS = game:GetService("ReplicatedStorage")
local remotes = RS:WaitForChild("Remotes")
local place = remotes:WaitForChild("Place")

local block = RS:WaitForChild("Part"):Clone()

local player = game:GetService("Players")['LocalPlayer']
local mouse = player:GetMouse()
mouse.TargetFilter = block

local building = false

local grid = 5

local function round(pos)
	local flooredValue = math.floor(pos)
	return (flooredValue - (flooredValue % grid))
end

game:GetService("UserInputService").InputBegan:Connect(function(key, typing)
	if key.KeyCode == Enum.KeyCode.Z and not typing then
		building = true
		block.Parent = workspace
		game:GetService("RunService").Heartbeat:Connect(function()
			block.CFrame = CFrame.new(round(mouse.Hit.p.X), mouse.Hit.p.Y + block.Size.Y / 2, round(mouse.Hit.p.Z))
		end)
	end
end)

mouse.Button1Down:Connect(function()
	if building then
		place:FireServer(block.Position, block.Orientation)
	end
end)