Localscript Depending on Client FPS?

Hello, I am trying to make a part move up and down at the same speed for all players, but this localscript makes it so that it changes speed depending on the fps, slower fps makes it slower and faster fps makes it faster.

local RunService = game:GetService("RunService")
local part = workspace:WaitForChild("LollipopUnion2")
local loopNumber = 200
local length = 50

while true do
    for i = 0, loopNumber, 1 do
        part.Position = part.Position + Vector3.new(0, length / loopNumber, 0)
        RunService.Stepped:Wait()
    end
    for i = 0, loopNumber, 1 do
        part.Position = part.Position - Vector3.new( 0, length / loopNumber, 0)
        RunService.Stepped:Wait()
    end
end