Don't even think about using a push button to activate the nitrous. It will only be a matter of time before you kill parts like that. The only way to activate N20 correctly is to use a WOT switch. They have some TPS WOT switches out there but I have never had much luck with those. The traditional microswitch type WOT switch activated by the bell crank on the TB is the only way to go. You should also get yourself a RPM activated window switch. Do not spray below 3000 RPM.
I would also suggest that you get your self a liquid filled N20 bottle gauge and a bottle heater to monitor and maintain proper working bottle pressure within the range that Zex suggests. It's not critical, but I would also get a FPSS (fuel pressure safety switch).
There is no reason to pull any timing at all at WOT for a small 75 shot, but it wouldn't hurt to either get a flip chip with a N/A tune and N20 tune, or just have the car tuned to make better use of the nitrous. For what it is worth, I am running full total timing with my 125 wet shot with not a single hiccup.
Did Zex include a relay for the arming switch?