I still use only my unaided eyes, but things aren't as sharp as they
once were, so it'll be a matter of time before I'll have to start
thinking about assistance. I tried using a magnifier, but found the
same problem you did. Takes practice I guess.
I used to use a 10/0 for most of my face painting when I was still
doing mainly 1/35th scale figures for armor dioramas, and I have an
18/0 which I use for only the smallest details now, but #1 is the
smallest brush I use for most painting. I can get fairly fine detail
with it, but then I work in larger scales mainly these days. However,
either someone told me once, or I read it in an interview with a
painter that a good painter should be able to paint whatever he/she
needs to just fine with nothing smaller than a #1. I feel that whatever
works is what you should use, and if it's a 18/0 then that's fine,
however, I've kind of taken that advice to heart and have used that #1
brush as my smallest for years, even on 54mm figures.
You'll have to try some stuff out and see what gets you the best
results. I would highly recomend that you get ahold of a copy of
Historical Minature Magazine and set up a subscription. They always
have great articles on painting (in both oils and acrylics), making
groundwork, sculpting original figures and such, plus tons of great
color photos from all of the big shows around the world. A great
resource for learning and for inspiration.
http://www.r-kproductions.com/historical_miniature_magazine.php