I have been useing this guide since reading Chajing by Lu Yu and The Story of Tea by Heiss.
Being an artist I am very visual so to me this is easier than getting out a thermometer.
1st boil = shrimp eye, lots little bubbles forming at bottom of pot, rising slowly, sounds like bamboo leaves swaying in strong wind
2nd boil = crab eye, medium size bubbles travel up fast, loud popping noise
3rd boil = boiling, fountain like water surface, no more bubble, constant humming noise
I hope this works as well as I has for me, for your own tea brewing.