The research window may look complicated or confusing at first glance, but it was made this way for a very important reason. Theoretical and Practical Research Ability and Strategic Effect Money in Hearts of Iron 3 is very important, and quite essential for trading. At maximum funding, this will increase your leadership total by 20%, while greatly reducing your available income. The most effective way to raise available leadership is to raise education spending.
It seems to be largely based on the overall University, Military Training, and Industrial Technology level of the nation in the mid-1930s. The US starts with low 20's, and the UK has a base around 18 or 19. It does not seem to be affected (at least not largely) by your total industrial power. I have not discovered what all the factors are in a countries BASE leadership value is.