App to practice in throwing tecnique

I'm not sure about another app, but I'd recommend that you go hunting for Zubats and practice your throws on them, as they're also far away.

Catching Ray is just like ho-oh/lugia. You need to throw ball vertically across the screen. Start spinning ball like normally and release ball on the right side of the speaker/brand logo.
I have been throwing excellents since 2nd or 3rd Ray raid.

Ps. I don't know app like you describe

I don't think there is an app like that, but someone really should make one. Perhaps have other helpful features as well that will help you become better and optimise your playing.

I had this problem with the firsts Groudons. It was the first legendary too far away for my curveball lands (I stopped playing and came back on Ho-Oh raids, so I missed Moltres and other far away mons).

I live in South America, so I practiced my throws on Heracross (always trying excellent curveballs) and now didn't have problems anymore, even with Rayquaza (missed only 1 throw on him, as my phone lagged).

So, my advice is practice with faraways moms like Zubat, Gastly's family, Heracross (in case you get the chance to) and so on.

I've advocated for some time for a "training space" where you can pick attackers and defenders and practice timing against different move sets or throwing, as you describe. Torchlight has a dummy in towns that you can practice on. PoGo would benefit from something similar.