It all depends on work done to the car. I will assume pretty much stock. I would say 75 shot is about as high as you would want to go. As far as SAFE wet kit.High side on fuel you wont lean out ever if you go low side without tuning you may lean out to much and BANG or BOOM witch ever you prefer. And use one step colder with your plugs in a zetec i use autolite 4133 if you run the same size plugs that is what you will want only like 2 bux a set. The same if you go dry, Dry kit will require bigger injectors and and chip to change the fuel load for the injectors. Go wet or you will end up going home in a tow truck with a fat guy named frank.
Hey man its cool i should of discribed it a little every nitrous kit has different nitrous and fuel jets these are the little nozzles.Now to achieve say a 50 shot you will need a fuel jet and a nitrous jet.The nitrous jet will remain the same but on the fuel side you would have 2 choices it is for fuel pressure.I think most company's use 35 psi and 55 psi.If you use the 55 psi setting the "high side" you will be safe.It is a larger jet so you get more fuel.
The NX kit is the best I've ever used. The provided jet sizes keep you plenty rich at all times. Wet is the only way to go. Use a wot switch, a fuel pressure switch, and an rpm window switch and you'll be virtually bulletproof. The general rule of thumb with nitrous applications is, most any engine can withstand a 50% hp boost from stock worth of nitrous. ie: 100hp zetec can take a 50 shot safely. I'm running a 75 shot with no trouble, it's tough to stick with 50 when 75 is just a wrench turn away.
One degree of timing should also be taken out for every 50 hp, but it's fairly safe to go a 50 shot without, 75 is pushing it, 100 will be trouble. The only way to retard the timing so far on our cars is with a chip. hope this helps, good luck
oh you might want to look into the NX direct port and NOS noszle system...they didn't fit on the zetec, but they might fit in the duratec given the different engine layout. You need to have room for a one inch spacer between teh fuel rail and the head.