Two points:
Nothing beats a good brush, and a single GOOD brush with care will last you many years.
I like size O or OO brushes, and I prefer Robert Simmons with the big black handle avaiable at Hobby Lobby or Michaels. Clean it carefully and onl use it for eyes or other very fine details, use hair conditioner on it once in a while and it will last many years.
In 28MM and larger try blocking in your eye color with off white or very very light pink inside the eyeball area with a "two -stroke- per- side of the eye"movement that looks like this ---< O > --in reality when you are done . Fill in the iris later with your color of choice using brown, blue orhazel and finally a black pupil. Shep Paines work in his books demonstrated the cross principle but I was never able to pull it off successfully so I use this instead and it looks good. Also, keep in mind you will almost never see the entire iris unless the upper eyelid is totally pulled back either in fright or anger, whcy may be appropriate here, BUT, you dont want a popeyed look like all my first figures were.
Secondly I read many years ago (and it has served me well) to paint eyes on a figure while holding it upside down, somehow it seems to keep from painting the figure cross eyed.
I dont always have my figures looking ahead, i like ot make them look off to the side a bit toadd visual interest. If all else fails , paint the eyeball off pink of oyster grey, and use the very nice Archer Human Eyeball decals.
HTH
David
Build to please yourself, and don't worry about what others think!
TI 4019 Jolly Roger Squadron, 501st Legion