What do you mean how high the NPK level can go?
There is no such thing as level...there is only ratio therefore if you are happy with 4-3-8 then stick to that ratio, which simply means 4 part Nitrogen, 3 part Phosphorous, and 8 part Potassium.
There are many ratio's. Such as 1-1-2
or you can go crazy like random ratio 50-1-100. You can dump an entire bag of Nitrogen to 1 tsp of phosphorous, to 1 tbs of potassium....its still called a ratio.
But I think you're confusing Volume or PPM with NPK or the other measurement EC which Michael have pointed out.
Seriously, lets take your NPK ratio of 4-3-8
example;
4 tsp of N, 3 tsp of P, 8 tsp of K
4 pounds of N, 3 pounds of P, 8 pounds of K
Both have the same ratio of 4-3-8
The difference between the two is in volume or
if you gonna mix those in water you have PPM or EC