hi mike welcome to the forum, have a look at he introduction section and maybe consider telling us a bit about whoyouare and how you arrived here..... we are a friendly bunch.....
so as for tips on your eye:
slow down, work the trigger of your brush smoothly and gently,
build your picture in layers, so for example, you have a nice light grey around the iris, paint the whole picture with the lightest tone, then get a bit darker and paint all the darker bits and then darker still building slowly in layers is the key.
paint what you see..... look at the finest detail of your reference, rather than focusing at the whole picture as 1 maybe consider looking at 2 inch square, and painting everything you see i that square ( background texture like wrinkles and pores etc)
hope this helps..... by no means have you done badly its a good start and you will quickly improve and suprise yourself..... i did
here is my first eye that I did just over 8 months ago