555 timer control pin help

i need formulas or something telling me how voltage change effects the period on a 555 timer

everywhere i look the websites keep telling me that it is usually disconnected or attached to a capacitor to ground

also is there a free circuit simulator somewhere on the internet i have been using this site
Circuit Simulator Applet
but today i have found out that the maximum frequency on the scope is only 16kHz and i need to measure between 25kHz and 60kHz

i need to select components for a 555 timer to make the output frequency at min 25kHz and at maximum 51kHz
but the period is determined by the control pin

dont worry i have figured it out the circuit simulator can show higher frequencys all you have to do is click on options then other options and change the time

and through trial and error and the basic formula i have solved it