Optimal Black Hole

Anyone have a quick guide on how to set-up Black hole with Golden Tower? I have my Golden tower maxed besides the final few upgrades to the duration that are exceedingly expensive. I just unlocked black hole and I know there are some optimal setting for it, but also that changing the range of it (size?) beyond a certain point ruins it for good. Just looking for a little help. I know I get the coin lab and also match the duration to the GT and no lower/no higher. What should the size be? And should I upgrade to have more of them? Thanks for any help!