If it simply had one vacuum line going from the turbos' hose barb fitting to the side of the wastegate, then the previous owner was adjusting the boost by the spring inside the wastegate. That type of install is what is called Open Loop method. Meaning the only thing that controls the boost level is the actual spring pressure that the spring inside the wastegate has. Some external style wastegates have an adjustment screw on the top of them, some don't. You can see on your ATR wastegate that it has a set screw with a jam nut. Tightening in on that set screw (Clockwise) effectively increases the spring rate, which will raise the boost. Some like to do this, some don't. Without having a good boost gauge inside the car and relying on the iffy at best stock LED boost lights, is disaster waiting to happen. I've seen the factory LED lights off by as much as 5-6psi before.
Get a good aftermarket boost gauge, mount it where you can easily see it during spirited driving, then install the manual boost control valve like the drawing above. With the vacuum lines hooked up like the drawing, that method is what is called closed loop. The adjustable valve routes the pressure that would normally vent to atmosphere, back to the top of the wastegate. The more pressure you put to the top of the wastegate, the stronger the spring rate will become. Effectively raising the boost level. When the adjustable valve is completely closed, this setup reverts back to spring only controlling the boost level. With the closed loop method, it is best to have the adjustable valve completely closed, set your base boost level with the adjustment screw. Once the base level is set, then you slowly crack open the adjustable valve during a WOT blast until your desired boost level is reached.
Typically I would set the base at or around 17-18psi when the valve is closed, then adjust the valve accordingly for the desired boost level. Don't forget to tighten up the jam nut once you have your base boost level set screw set. Just pay attention to your driving when adjusting the knob as things happen very quickly and don't go overkill on cranking on the knob. It usually only takes a half to 3/4s of a turn on the knob to start raising the boost. With practice, you will get the feel of the valve and the tension that is required on the knob in order to raise the boost. There is a slight amount of slack in most of the bleeder valves like the one pictured in the drawing. You will feel this slack when turning the knob.
What I've found is that once you start to feel this slack tightening up, that is the base point of adjustment. Meaning that from completely closed until that base point, nothing happens to the boost level. However, every boost valve is different, you will need to experiment/test the valve you have. Just don't get crazy with twisting and twisting the knob. No sense is blowing a head gasket while setting your boost. If while setting the boost, the boost runs away, just lift. Check your connections and retest. It may be necessary to port the turbine housings' wastegate hole to the same ID of the tubing that feeds the wastegate. This is especially true with P-trim turbine wheel equipped turbos.
Hope some of this helps a little.
Patrick