Draw power is certainly one factor, but so is individual card count. A 32 card side with 4 copies of 8 distinct cards is going to deliver a much different experience than a 32 card deck with 2 copies of 16 distinct cards.
It's also helpful to look at cost of cards on the Shadow side. If the Freeps is giving you an average of 7 TL per site in your current meta, it seems to me that the total net TL cost of your Shadow side should be around ~56. That's assuming you cycle through your entire deck, which rarely happens, of course. Most of us are not comfortable with that idea, so it's better to have a bit more than you can play in a single game.
With that in mind, I tend to build like this:
32/32 baseline
For every 3 cards I can draw during a game, I can add 1/1
So with 1-2
Elrond LoR I will play a 33/33 deck. Add an
Abandoning Reason for Madness and I'll probably go to 34/34. Or, With 4x Goblin Scimitars, 4x
Goblin Scavengers, 4x
Host of Thousands I will play 36/36.
Relics of Moria and
They Are Coming give me more wiggle room, but I don't always
TAC or HoT for a Scavenger. But toss in a Bilbo and Redbook of Westmarch combo or more Elronds or
Gandalf the Grey Pilgrim and I easily get to 40/40 for Swam. I also have a turn one Freeps filter which changes the mix of my deck dramatically, so search cards should be considered as even better than draw power with certain decks.
That's how I think about it, anyway.