Someone please tell me I'm not the only person churning out Level 50 Enchantments resulting in Efficiency IV pickaxes? I've now had six in a row. The tears. They taste so salty.
I would recommend enchanting at level 31. This is for the best results efficiency wise. It's not too high of a level to get to, yet you can still crank out those good picks!
I suggest level 48 for those Pickaxe's with three enchantments. E.g. Silk Touch I, Unbreaking III, Efficiency IV. You won't get this all the time but when you do it will pay off.
What is worse is getting Unbreaking III at lvl 50, it happens to me all the time. I will usually enchant picks at 47-48 and shovels and axes at lvls 45-46 just seems to work for me.