I think you can start at 50mcg ed for a week or 2 and then bump it to 100mcg.
I don't even think you would need that much as it may be catabolic but depends on your goals. I believe your t4/t3 levels don't decline by more than 30% while on gh so you would need just enough to compensate for that.
I started my cycle 4 weeks ago and started immediately with 50mcg of t4 but 2 weeks ago I switched from t4 to t3 at 12.5mcg ed. That seems to be perfect for me as there is no change in my waking temperture.
Just remember if taking T4, specifically synthroid, take them at the same time everyday( pref morning ) 45-60 mins before a meal. Hope that help, Cheers.