since there's no projection in 2D, screenX/Y() is (should be?) all you need.  note though, just as with modelX/Y(), they return the total of all transformations currently in effect - only potentially an issue because your translate is outside your push/pop, but would be reflected by screenX/Y().  that is, if you were to grab those coords just before popping the matrix, you'd get:

 

ellipse(100, 0, 10, 10); // for reference

float sx = screenX(100,0); // 300

float sy = screenY(100,0); // 400

 

if you instead grabbed them just after popping the matrix, you'd get:

 

popMatrix(); // for reference

float sx = screenX(100,0); // 400

float sy = screenY(100,0); // 300

 

note that in both cases the initial translate(300,300) is still in effect, you'd have to push/pop around that too then call screenX/Y() after that pop if you wanted coords relative to screen center.

 

hth