225 Slant Six Crank but Won't Start (Fuel but No Spark, Dist. Rotor turns)

I'm not sure how comfortable I am with running the starter long enough for me to figure out the timing with a light gun. ( remember I am still a newbie ). Plus I don't have a light gun available to me I would have to go rent or buy one.

But could you please explain to me the difference between TDC and TRUE TDC? I am fairly certain I know how to find TDC of the compression stroke but what is TRUE TDC? I keep reading posts about finding tdc then turning the pulley the opposite direction until the cylinder comes back up on the compression stroke again. Then deviding the two marks to get true tdc?

I am sorely confused on why I need the extra steps?