I'm thinking of Prone Y/I Raises in particular.
I think I once read somewhere that having a 120 degree abduction is most effective for engaging the supraspinatus, though I'm not sure how true this is.
When I do these exercises, I think I can feel them working my supraspinatus, but I could be getting confused with something else.
There is so much going on at the shoulder during any type of overhead and rotational movement.
The supraspinatus is a smaller GH stabilizing muscle that never works in isolation - so this may be one of many muscles you are feeling.
The contracting muscles are dynamic related. With a line of pull (or force vector) that is constantly changing and it's involvement during movement is heavily tied to it's position at any given time.
The exercises you mention involve shoulder flexion (and therefore high anterior deltoid activity. Below shows the significance of the 120 degrees your friend mentioned.
If you'd like more detailed info on any part of this let me know.
Yes those exercises would work fine. The Y being a better choice than the I.
Also just curious what is your goal and why are you trying to target the surpaspinatus?
Ideally to get the most "bang for your buck" I'd use a more function pattern (consisting of a diagonal and rotational pattern) such as D2 Flexion. Some of the best shoulder injury prevention and rehabilitation programs consist of variations of these exercises.
UPDATE #2
Are you sure it's the supraspinatus?
If you can see a PT in your area, however this is a simple clinical test that used to test for supraspinatus lesions:
