Universes and ports do not have any direct correlation. A port can start on any channel in a Universe. Universes are just a method to be able to identify each pixel so that every component in your system knows where each pixel is, including you. (In reality, Xlights and the controllers will just identify them by a sequential number, called Absolute Addressing)
You can start your Universes with any number, make them any number of channels (up to 512) and even skip universes. It just has to make sense to you but you also need to be able to let your other systems know what pixels they are responsible to control so you should be careful in how you setup your universes. There are a few standard methods but some people follow their own method. I personally have my universes grouped in series of 100. So my first Universe group starts at 100. Each Controller will have its own universe group so my first controller will start at Universe 100 channel 1 (I use 512 channels, but 510 is more common) and setup just enough Universes to handle the Pixels that are being controlled by that controller (this might need to be adjusted if you leave blank channels).
So my first controller might have Universes 100 to 115. The next controller will start at Universe 200 and then setup however many Universes I need for that controller. So my setup page will have Universes 100-115 and then Universes 200 to however many I need for that controller. In Xlights, I setup my first model that is going on the first controller and first port to start at Universe 100 channel 1, and then if there are other models that are chained in line, then the next model will have its start channel to be setup as a relative address to the model before. (see attached pic) I don't manually set each model with a start channel. Let XLights do the management for you! Then when I start on the next prop, I will just see what Universes were used on the first port and start that model with the next Universe, channel 1.
In regards to your MegaTree, just start it at Channel 1 of whatever Universe you have setup and let it increment from there. Like I said before the ports do not have a direct correlation to Universes. Just tell XLights the total number of strings you are using (Strings ARE the number of ports you want to use on your controller, so in your case, it would be
Then you will tell XLights how many Pixels are connected to each port (Nodes/String) and then you tell it how many times the strand connected to the port goes up and down (strands/string)
Let me try an example that is close to what you are doing. Assume that you have a MegaTree with 16 strands of 75 pixels on each strand for a total of 1200 Pixels. Each String will be 4 strands on the tree so 300 pixels on each port.
With that you will setup 8 Universes. Let's say Universes 100-107 each 510 channels. Then all you have to do is go to the controller and say that output 1 will start at Universe 100 channel 1 and have 300 Pixels. The controller will tell you where this string ends, in this case Universe 101 channel 390. So then you would tell the controller that Port 2 will start at Universe 101 channel 391, etc. (Did I tell you that if you use XLights to its full potential, it will actually setup your controller for you?)
I attached a pic of my controller for an example. (I use 512 channels/Universe)
