As a rule, what is the ideal % protein in a pre-mixed feed?  What do you consider "too low?"  What do you notice when a horses isnt getting enough protein or too much??

Well, I'll give this a shot. 

There is no direct answer to this question because you need to look at feed differently. The Protein needs of a horse depends on maturity and use. Most protein is provided by the roughage, since that is, or should be, the largest part of the diet. Grass hay can vary from 6% protein to 16% protein. So, the answer to whether or not the roughage is providing all the protein that your horse needs can vary greatly. 

Growing horses should get a minimum of 15% quality protein in their total diet to allow proper growth. Mature horses can do great with around 11% total protein in the diet.  Mature performance horses can use a little more protein, 12% to 14% is plenty.

Alfalfa hay usually runs from 12% to 24% depending on quality. I would avoid the upper end if feeding any alfalfa, but you can see that a blend of the two can usually provide all the protein that even a performance horse needs with no additional feed added. This makes sense since there was no place in nature that the evolving horse could stop at the grain pile to get the protein that they need. 

Still,  if you are feeding very poor quality hay, you may well be low on total protein in the diet. This can be a place for a "Ration Balancer" to up total dietary protein. These run around 30% and, in my opinion, feeding this with very poor hay is about the only proper place for this kind of product. An easier solution is better hay, or "some better hay" added to the current poorer quality hay or pasture. 

So why add processed feed at all? Small additions of concentrated  feed that provides higher fat levels, fat soluble vitamins that a horse can't make themselves, or specific mineral needs will better fit the horses digestive system and can provide needed support without disrupting the hind gut with too much starch and sugar. Why upset the whole system when you are just trying get the protein you need. Let the roughage provide the protein if at all possible, and supplement for other needs in the smallest package possible. The bottom line here is that better hay is always a better solution to higher protein needs. So the answer after all this, is that there is no way to find ideal protein percentage in processed feed that gives your horse a balanced diet until you know how much protein is already in the diet from your hay and or pasture.