Like the others said, a set of manifold gauges would be significantly better than what comes with the can of refrigerant. Whatever you do, don't buy any stop leak or "miracles in a can" because it's only going to give you a bigger headache than you had with a leak in the system.
On the WRX, I wouldn't be surprised if your leak is coming from the o-rings where the lines meet the compressor. Super common problem I have seen on these cars. Hopefully that's the case and you wont' need to buy a $500 compressor