Creating smooth part rotation and animation?

I have tried making a smooth animation that will always start smooth and stop smooth. When I try to go from 0 to 180, it stops quicker then how it started. In the reverse, it starts quicker then how it stops.

function STRotate(to,speed,set,con)
	local to=to
	local from=set.Rotation
	local doNEG=1
	if from>=to then doNEG=-1 print("fliper") end
	local sin,rad=math.sin,math.rad
	for i = from,to,doNEG*4 do
		print(from + sin(rad(i))*(to - from),"--",from,i,to)
	    set.Rotation = from + sin(rad(i))*(to - from)
	    wait()
	end
	wait(6)
end

while wait() do
	print("ONE")
	STRotate(90,.05,script.Parent.Frame.Back)
	print("TWO")
	STRotate(0,.05,script.Parent.Frame.Back)
end

EDIT: I now have a different code, but for some random reason, I can't make it stop getting a really big negative number on the first three results.

-- For all easing functions:
-- t = time == how much time has to pass for the tweening to complete
-- b = begin == starting property value
-- c = change == ending - beginning
-- d = duration == running time. How much time has passed *right now*
local function inOutBack(t, b, c, d, s)
  s = (s or 1.70158) * 1.525
  t = t / d * 2
  if t < 1 then return c / 2 * (t * t * ((s + 1) * t - s)) + b end
  t = t - 2
  return c / 2 * (t * t * ((s + 1) * t + s) + 2) + b
end

function STRotate(to,speed,set,con)
	local from=set.Rotation.Z
	print(from.."----"..to)
	if from>to then
		local holdfrom=from
		local holdto=to
		to=holdfrom
		from=holdto
	end
	local timer=4
	for i = 1,timer,.05 do
		local num=inOutBack(timer,from,from-to,i)
		print(timer," \ ",from," \ ",from-to," \ ",i," \ ",num)
		script.Parent.Rotation=Vector3.new(0,0,num)
	    wait(.1)
	end
	wait(2)
end

while wait() do
	print("ONE")
	STRotate(20,.01,script.Parent)
	print("TWO")
	STRotate(80,.01,script.Parent)
end