Listing rules of thumb: Need contributions

I'm a baby when it comes to growing/harvesting however I feel that having a list of basic rules of thumb would be beneficial for everyone.

For example:

1) 4:1 weight ratio when comparing the weight before drying and the weight after drying.
2) For everyone 1 foot of plant you should have 1 gallon of soil space.
3)... hopefully we can continue the list!

Thanks

4) Never let your pets have access to your grow room (especially cats)