Laying out the LED soffit lighting:
I'm no electronics guru.. but I have a bit of experience with LEDs. I've probably soldered over a thousand of them to go with my 200+ channels of computer controlled Christmas lighting. I understand the basics.
I've seen some people try to do some pretty crazy things with LEDs, but you really have to put some thought into it to get even lighting that works.
The key here is voltage drop. If the voltage goes too low you will get dim LEDs. Worse, depending on how you wire it, you could end up with some bright and some dim. Industry standard for most things says that allowable voltage drop is 5%. With LEDs I go for 3%. That is, 3% drop in the wire that is supplying the strips. That's because the voltage will continue to drop in the strip itself. How much? I don't know, because the trace widths and thickness will vary with manufacturer. It's easy enough to test, just power one end and read the voltage at the other.
So I'm shooting for a 3% voltage drop, or less than 0.36 volts (12 * .03)
I started by selecting the LED strips I am going to use. I just want red (not RGB), so I picked the 3528 LED strips with 60 LEDs per meter. Specs say these will draw anywhere from 4.7 to 5.3 watts per meter. If you have fewer LEDs per meter, they will draw less current. It's important to know what you are buying!
I knew I wanted to use 18 gauge wire.. because I had a ton of it. Thermostat wire is typically 18ga and it is rated for in-wall use. I had a spool of 18-4 thermostat wire lying around.
Next I needed to figure out the voltage drop using 18ga wire. Now, if you didn't have any wire, you could work the other way.. what gauge wire do I need to keep the voltage drop within parameters.. but it could get expensive quickly, and the thought of running 14 or 12ga wire to strips is not a good one.
Voltage drop is a result of the length of the run, the gauge of the wire... and the current being drawn. Draw less current and you have less voltage drop.
Using the diagram of my ceiling as pictured above, I used 2.77 meters as a starting point. Specifications for the strips will state they draw anywhere from 4.7W/m to 5.3W/m. At a worst case of 5.3W/m (0.44A/m) the 2.77m strip would draw 1.22 amps (0.44 * 2.77). I figured 20 feet as the longest length of my 18-4 that would supply the strips. You can punch these numbers into any of the online voltage drop calculators and you will see this yields a voltage drop of 0.31V. 0.31V is less than the 0.36 I was shooting for (3%), so this is a good match. If I make the strips longer (ie combining a 2.77m and a 1.52m piece), I'll draw more current and the voltage drop will surpass the 0.36 threshold-- no good.
Consequently I ran an 18-4 to each corner as depicted by the magenta lines in the diagram. I will take 2 of the conductors from the 18-4 and power a single 2.77m piece of strip LED, and I will take the remaining two conductors and power the adjacent 1.52m strip.
A few caveats you need to keep in mind.. The traces on the strips are only able to carry a certain amount of current. Draw too much current through them and they will burn out. Check the description for the maximum length permitted. Second, keep in mind that with RGB strips all of the current is returning on a single conductor. You probably have four wires going to the strip, 1 conductor is +12v for red, another +12v for green, a third is +12v for blue and the last is a shared -12v for all three. To put it in household terms, you wouldn't take 3 circuit breakers, run a black wire to each, connect a receptacle to each of the 3 black wires and then use a single white wire for all 3 receptacles. You have to sum the current to size the white wire. Typically you will be running red OR green OR blue, so it may not be a problem. However, if you start color mixing, or turn them all on simultaneously.. you could have a problem
To control my red strips I am using a GRX-TVI interface with the Grafik Eye system. I picked it up brand new of the bay for some crazy price like $20 a while ago. That will interface with a Meanwell HLG-240H-12B 12 volt power supply. The important part is the "B" at the end. That means it accepts the 0-10v control the GRX-TVI will output to control the LED dimming.
Again, you have to size the power supply properly.. Using our worst case of 5.3 watts per meter, multiplied by the total length of my strips (25.74m) I come up with a total of 135 watts required.