It really all just depends on the quanta demand of your deck, and how many elements you are using. A good starting place is 40%. That's 12 towers in a 30 card deck. If you don't need much in the way of quanta (cheap creatures and spells), you can go with one or two fewer. If you have expensive stuff, it would be better to up to 50%. Some decks even take 60% towers. You can replace some towers with quanta producing creatures too.