how can i snap an uneven size part to grid in my placement system?

long story short im creating a grid placement system with snapping into place, but it only snaps if the part size is even (for example 2x2x2). if its not even then it snaps between the grids. i have tried adding half a stud to the part so its in the right place but it didnt work well. here is the script and an image:

local module = {}

module.place = function(plr,item)
    local blacklist = {}
    for i,v in pairs(item.Parts:GetChildren()) do
        table.insert(blacklist,v)
    end
    local mouse = plr:GetMouse()
    local function snapToGrid(vector3,middle,folder)
        local x = math.floor(vector3.X+0.5)
        local y = math.floor(vector3.Y+0.5)
        local z = math.floor(vector3.Z+0.5)
        folder.Parts:MoveTo(middle+Vector3.new(-x,y + folder.Parts.PrimaryPart.Size.Y/2,-z))
    end
    local hitbox = item.Parts.PrimaryPart
    local RS = game:GetService("RunService")
    local maxHeight = 250
    local plot
    for i,v in pairs(game.Workspace.Plots:GetChildren()) do
        if v.Owner.Value == plr.Name then
            plot = v
            for i,b in pairs(v.BaseParts:GetChildren()) do
                b.Grid.Transparency = 0
            end
        end
    end
    item.Parent = workspace.Placing
    RS.RenderStepped:connect(function()
        local ray = Ray.new(mouse.UnitRay.Origin,mouse.UnitRay.Direction*1000)
        local hit, mousepos = game.Workspace:FindPartOnRayWithIgnoreList(ray, blacklist)
        local pos = plot.BaseParts.BasePart1.Position - mousepos
        if hit then
            snapToGrid(pos,plot.BaseParts.BasePart1.Position,item)
        end
        wait()
    end)
end

return module

http://prntscr.com/xihm1t