I open the chain with a chain tool, slide my wheel forward in the dropouts, then use the chain to figure out where the new break needs to be. The break the links off, put the chain together and check the length.

Or, do the math:
http://www.rohloff.de/en/technology/.../chain_length/

Tension: put your wheel on with the chain sort of loose. Spin the pedals forward and backward. Burn that ease of spinning into your muscle memory. Now make the chain as tight as you can without cutting down on the ease of spinning.