http://spikerlights.com/calcpower.aspx
Nifty tool that lets you input all of the variables like number of LEDs, wire gauge, voltage, wattage and spit out a graph showing the results.
If you plan on running lots of LEDs, this tool will save you much trouble by telling you early where you should inject power to keep things in the green.
So many thanks, I really needed this … I have 832 pixels with power injected at:
pixel 0 2.25a
pixel 300 (via 6m extension) and pixel 600, 8.0a
pixel 900 (because I’m not using the end of the strip), 2.25a
and I’m trying to figure out how to get full power around pixel … 400 or so.
Looking forward to using this tool… and I don’t need things in the green, I need 'em in the blue, because the power drops off on the blue LEDs first. 
This is a wonderful tool!! Thank you for sharing