nitrous on a b16

I have an 89 crx si with a b16 a i recently put in and i wanted to get alittle more pwer so i bought a zex nitrous kit and zex spark plugs. The kit will do a 35 55 75 and 100 shot. i started with the 35 then went to the 55 and it runs great. My b16a has only a few mods fuel rail, intake, exhaust, ect... and i was woundering about retarding the timing, i read somethings like 2 degrees for this and 4 for that, what is the degree of timming for a 75 and 100 and also how do i know how much i have retared the timming because my distributer has no marks?

thanks

theres no set amount of timing to retard because every motor like different timing, some needs more while others need less. but usually more retard is better so you dont come close to blowing the motor