I came across this 555-timer calculator that is awesome in my opinion. It will let you calculate Astable, Monostable, and you can get the values of the resistors, cap, and time on/off.... It gives you the correct values based on what information you put in. Here is an example of a Astable circuit, if you put in 5 sec "ON" and 2 sec "OFF" then it will tell you R1=19.7k R2=13.1k C1=220uF, that one is based on the voltage I use which is 5v which you also enter. It is only a 900k download and it’s free. I have no affiliation with that site, maybe it belongs to someone here? http://clarkson-uk.com/555-timer/program.html
