A new downloadable varroa guide is available from Veto Pharma. The percentage of infestation to treatment seems a bit on the low side (others suggest treat at a 1-3% infestation) but still a very helpful reference and explanation
http://www.veto-pharma.com/wp-content/uploads/2016/07/Guide-Varroa-USA-2016.pdf
Another great tool for varroa management put out by the Honey Bee Health Coalition can be downloaded here:
http://