PWM buffer to adjust ignition timing

First off, I am an electronic noob, but I am an automotive technician, so my language base is in cars. Please try to reply in more layman's terms.
My problem is I am putting a turbo on my car and need to vary the timing under boost, the signal from the ecm to the ignition module is a 5v pwm square wave signal, I need to put in a circuit that will allow me to adjust the pwm signal to adjust the timing using a potentiometer and the circuit needs to be able to be bypassed when not under boost.

Do you need to advance or retard the timing under boost?

retard it to prevent detonation, stock timing map is pretty aggressive. on a stock engine if the egr stops working you can hear the pinging from the timing being too advanced. if I could adjust the timing enough to eliminate the egr that would be even better.

So, should both the rising and falling edge of the ignition signal be delayed by an equal amount?

Also, should the amount of delay be a function of RPM, or does the ECM already take care of that?

the ecm sends a 5v pwm to the module to tell the module how much to advance the timing, the crank sensor connects directly to the module, if the signal i am trying to modify turns off, the module goes into base time (limp home mode)
i believe that i need to adjust the pwm%, i think if the ecm sends a 70% signal i might need to turn it down to 50%,this is a guess i will have to do some testing to see what i am dealing with. i don't know if it varies voltage and on/off time seperatly. i have read on the megasquirt website that the pwm is a square wave, i don't exactly know about the rising and falling edge part. but there is also dwell control somewhere in there too.