A rewired Walboro 190 will flow about 162 L/hr at 58 psi (aka 21 psi boost in a GVR4)
This equates to 2700cc/min. This is 675 cc/min per injector at peak flow. Or 99% duty cycle. Basically this means your injectors MAX flow is the same as your fuel system. Not nessisarily a bad thing, but if you crank the boost up any more, you're injector's will be able to outflow your pump = bad. If you don't know already I'd suggest researching how our fuel system works, and flow charts for your fuel pump.
Now, you don't want to run your injectors at 100% duty cycle. Basically that means they're just dumping fuel, with no control. Typically around 80% max duty cycle is desirable. That means the max fuel you want to flow is 544cc/min per injector, if your injectors actually flow 680cc/min.
What you need to figure out is how much fuel you need to reach your HP goals. You say "18-20 psi", what HP are you targeting? 300bhp?
Turbo Tech 101Read that and the next couple sections (102, 103). It's a great starter on turbo systems, and should help you estimate what psi you need to run to reach your goal, and how much air you will be flowing.
Also, once you set a HP goal you can use an injector calculator to size your injectors. (google it) Remember you want to target 80% duty cycle, and to leave some room for error's in your assumptions.