throwing to lugia would be like throwing to...

Just keep practicing. You were most likely nervous since it was your first time with him. Just adjust to how far he is and you should be fine

I can only say be exceptionally patient with your throws. ONLY release your ball if you have already seen Lugia attack AND you are certain that (judging from the size of the target circle) you can either score a great or excellent. If you hit a great every time you're bound to get it eventually > My 15/17 record for Zapdos was down to pretty much 60% of the shots being great and 20% excellent, but my 4/13 on Lugia was plagued with ball deflections, missing the target, not hitting great/excellent etc.

I wouldn't use another pokemon to 'practice' Lugia, but if you insist:

There is no bird with such a gigantic circle but I assume Pidgeotto (for closer throws like Arti) and Golbat (for further throws like Moltres) is always an OK approximation for legendaries except for Zapdos (as far as moving up and down and deflections but obviously it is no where near as hard for Pidgeotto and Golbat)

Do note Pidgeotto's hitbox is not on the head but rather near the torso, you should aim for the head if you want to hit it at the similar spot to Lugia. Use pokeballs and no berries so it increases the escape chance so you can have multiple throws.

Wild Ursarings seem to have the same size of circle like Lugia.