well that depends
some ppl like to just get a boost controller on there and turn it up silly and have no fuel upgrades to support it therefore when they stick on the fcd they end up running a bit lean and then boom! funs over
other ppl are prepared for a fcd and buy a walbro pump and raise the boost max to 12psi (depending on what mods) and install a fcd and never alter their boost controller
basically you need to tell us what mods you have in order to suggest when you need the fcd and what boost you can run as it varies alot
if you have a dyno nearby that would be even better but not everyone has those
or if you have spare money to burn buy a wideband so you can keep an eye on it
