"I have looked up the conversion from Naturthroid's website, but everyone says to go slow. "
That is your answer.
Whatever the chart says, i would go a little lower.
You have two choices in starting out Nature Throid. The TEMPORARY result will be medically induced hypo, or medically induced hypER. Its near impossible to get it just right as the body has to adapt to the increased T3 in Nature Throid.