need help with PWM

hi, im currently designing a switching power supply, the thing is that my input voltage changes about a +/-15%, so i need a PWM controlled by that voltage change, i really dont want to use IC's cuase im short of time, so i was thinking of using a 555, in the datasheet sais that the pulse's width can be controled with the 5th pin but there isnt a ecuation or anything at all to make my (teoric) design. Please if anyone knows how to calculate the PWM with a voltage change on the 555 let me know. Thanx

FM pin

Remember that the "mystery" pin 5 (fm, meant for modulating the output frequency) is a connection to the upper node of the input resistor network (& the negative terminal of the upper comparator), three ~5K resistors in series, providing the 2/3 Vcc and 1/3 Vcc trip points.

**broken link removed**