How to place a block with the mouse using mouse.Hit.Position?

I'm not having errors and it works, but it's really finicky. I have a LocalScript to do all the math and everything and then I send it through an event to place it with a ServerScript. I was hoping someone could improve my code to make it better. Here is the LocalScript inside StarterGui

local uis = game:GetService('UserInputService')
local plr = game.Players.LocalPlayer
local mouse = plr:GetMouse()

if uis.MouseEnabled then
	local OriginalMousePos = mouse.Hit.Position
	mouse.Button1Down:Connect(function()
		if plr.Mode.Value == 'Building' and plr.Block.Value ~= '' then --Mode is the mode they are in (None, Building, or Removing) and Block is the block they are using (string of the block's name)
			local pos = mouse.Hit.Position
			local x = math.floor(pos.X)
			local y = math.floor(pos.Y)
			local z = math.floor(pos.Z)
			local bs = plr.BlockSize.Value --Size of the block
			local base = workspace.Plots[plr.Plot.Value].Bases.Base --base they are building on
			local downX = base.Position.X - (base.Size.X / 2)
			local upX = base.Position.X + (base.Size.X / 2)
			local downZ = base.Position.Z - (base.Size.Z / 2)
			local upZ = base.Position.Z + (base.Size.Z / 2)
			local coords = Vector3.new(x + bs/2, y + bs/2, z + bs/2)
			if coords.X > downX and coords.X < upX and coords.Z > downZ and coords.Z < upZ then --Make sure they are in they're plot
				game.ReplicatedStorage["Remove Passive"]:FireServer() --remove any pasiive blocks on the base
				game.ReplicatedStorage["Place Block"]:FireServer(plr.Block.Value, coords, bs, false) --place a block
			end
		end
	end)
	while true do
		wait()
		if OriginalMousePos ~= mouse.Hit.Position and plr.Mode.Value == 'Building' and plr.Block.Value ~= '' then --If they still have the same position, there is no point in placing another passive block
			local pos = mouse.Hit.Position
			OriginalMousePos = pos --update OriginalMousePos
			local x = math.floor(pos.X)
			local y = math.floor(pos.Y)
			local z = math.floor(pos.Z)
			local bs = plr.BlockSize.Value --Size of the block
			local base = workspace.Plots[plr.Plot.Value].Bases.Base
			local downX = base.Position.X - (base.Size.X / 2)
			local upX = base.Position.X + (base.Size.X / 2)
			local downZ = base.Position.Z - (base.Size.Z / 2)
			local upZ = base.Position.Z + (base.Size.Z / 2)
			local coords = Vector3.new(x + bs/2, y + bs/2, z + bs/2)
			if coords.X > downX and coords.X < upX and coords.Z > downZ and coords.Z < upZ then --Make sure they are inside the base
				game.ReplicatedStorage["Remove Passive"]:FireServer() --remove any passive blocks
				game.ReplicatedStorage["Place Block"]:FireServer(plr.Block.Value, Vector3.new(x + bs/2, y + bs/2, z + bs/2), bs, true) --place passive block. 
			end
		end
	end
end

If you are wondering why I added half the size of the block when firing the event to place the block, it's to center the block on the plot.

If you need it, here are the scripts for placing a block and removing a passive. Placing a Block in ServerScriptService

local PS = game:GetService('PhysicsService')

game.ReplicatedStorage["Place Block"].OnServerEvent:Connect(function(plr, block, coords, bs, passive)
	local block = game.ReplicatedStorage.Blocks[block]:Clone()
	block.Position = coords
	block.Size = Vector3.new(bs, bs, bs)
	if passive then
		block.CanCollide = false
		block.Transparency = 0.5
		block.Name = 'Passive'
		PS:SetPartCollisionGroup(block, 'Building')
	else
		PS:SetPartCollisionGroup(block, 'Built')
		plr.Blocks[block.Name].Value = plr.Blocks[block.Name].Value - 1
		plr.Refresh.Value = true
	end
	block.Parent = workspace.Plots[plr.Plot.Value].Blocks
end)

Removing a Passive in ServerScriptService

game.ReplicatedStorage["Remove Passive"].OnServerEvent:Connect(function(plr)
	pcall(function()
		workspace.Plots[plr.Plot.Value].Blocks.Passive:Destroy()
	end)
end)

Thx in advance

Best bet is to not make it so difficult with your code. Obviously this is probably your first take with some fixed errors. Since instance positioning has an anchor right in the middle of the instance, you can split the instance into half, and then if there is a block below it or above it, simply half that too, then add those two halfed values and then position the new block wherever the sum position is at. No need to do math.floor and hacks for this.