Well love, this is a hard one as they say " how long is a piece of string" well as long as you want it to be, the same goes for earnings............
The more you know, the better you will be, the greater your service menu, therefor the greater your earning potential............
So in order to give your clients the best of your abilities I would suggest that your service menu covers all Systems, Gel, Fibreglass and L&P......
That way you can serve all and never have to say " I don't know how to do that".
Manicure knowledge is a must I feel, as you will learn all about the structure of the hand, muscles, nerves and such.........
Which will then stand you in good stead for you enhancements............
So it's a case of learn, practice, learn some more and then offer as much as you possibly can, because if you don't nail technician XYZ down the road will be..............
The Nail art service is for some a huge money maker but for others just a little add on service for the client......you need to research your area and see whats happening there, look at peoples nail and see how many are having fancy art or how many are just having french polish with maybe a little gemstone..........
Hope this has helped a bit
love Ruth xxxxxxxx