NPC MoveTo Player. How do I make the NPC keep at a certain distance?

im using a script to make an NPC follow the player. Im trying to make the NPC follow the player, but stay at a certain distance from the player. like this...

https://imgur.com/a/fXwOUdL

if the player walks away, the NPC gets closer, if the player gets closer, the NPC moves farther away...to maintain the same distance from the player

local Mroot = script.Parent.HumanoidRootPart
local h = script.Parent.Humanoid
local Mspawn = script.Parent.Parent.MobSpawn.CFrame

--Finds a player that is in range
--use "local player = findplayer()"
function findplayer()
    for _,v in next, game.Players:GetPlayers()do
        if v.Character then
            local char = v.Character
            if char:FindFirstChild("Humanoid")and char:FindFirstChild("HumanoidRootPart")then
                local pRoot = char.HumanoidRootPart
                if (pRoot.Position - Mroot.Position).magnitude <= 40 then
                    return v
                end
            end
        end
    end
    return nil
end

while wait() do
    local player = findplayer()
    if player ~= nil then
        local stayat = CFrame * CFrame.new(0,0,-20)
        h:MoveTo(player.Character.HumanoidRootPart.Position)
    else
        h:MoveTo(Mspawn.p)
    end
end

ive tried doing this...

while wait() do
	local player = findplayer()
	if player ~= nil then
		local stayat = player.Character.HumanoidRootPart.CFrame * CFrame.new(0,0,-20)
		h:MoveTo(stayat.p)
	else
    	h:MoveTo(Mspawn.p)
    end
end

The NPC keeps the distance away from the player, but it tries to stay directly in front of the player... like this...

https://imgur.com/a/GY57ohj

https://imgur.com/a/Iwk9mq2

how do i make the NPC walk straight at the player, but stop at a certain distance...and if the player gets closer, walk away from the player, and remain the distance