Tools
Low-voltage landscape voltage drop
The last fixture on a long run is where landscape lighting fails. Total load, gauge and distance in; volts at the end of the run out.
Voltage drop is what quietly ruins landscape lighting installations. Twelve volt systems have almost no margin, so resistance in a long cable run arrives at the far fixture as visibly dimmer, warmer light. The calculation is standard: two times the run length, times the current, times the wire's resistance per thousand feet, divided by a thousand.
The figure above is deliberately worst case, because it assumes every watt sits at the far end of the run. Real installations rarely look like that. Running a hub layout, where several short spurs radiate from a central point, or feeding from the middle of a run rather than one end, cuts the drop dramatically for the same amount of copper.
How much drop you can live with depends on the lamp. Halogen is unforgiving and shifts noticeably warm and dim below about 10.8 volts. LED landscape fixtures are far more tolerant, and most run happily anywhere between nine and fifteen volts, which is why multi-tap transformers exist. Check the fixture's stated input range rather than assuming, and remember that going up one wire size is almost always cheaper than a service call.
Before you rely on it
This is arithmetic, not a photometric calculation. It will get you to a sensible starting point and tell you when something is obviously wrong, but a real layout gets run in AGi32 against the actual IES files. Send me the plan and I will run it properly.