I'm not sure there is another, better, way, but I do have some reservation that researching a doctrine (staff studies, war gaming simulations, etc.) should result in immediate benefit. I'm sure that we can say that when it is done being researched that means it has been "proven" and implemented throughout your forces but even though this is a game that seems more than a bit artificial.
One thing that comes to mind is a "over time and use" method, where once you have researched a doctrine you then need to use it in combat to gain the full benefits of it (on the job training as it were). Since a new doctrine would involve staff studies and simulations before it was adopted there certainly could be some immediate benefit, but the maximium benefit should probably happen over time as that countries forces fully embrace the new doctrine. This could also be a method for first movers in a doctrine (Germany with Blitzkrieg for example) would build up a large doctrinal advantage over time while a "late comer" to that doctrine would not instantly gain the same bonuses from that doctrine.
So I could see that if country X gets a new Doctrine that they get a +5% bonus to something, and that as the appropriate units are involved in combat that this bonus slowly increases until it hits a maximum of, perhaps, +25%. There could also be an increase over time for countries at peace representing training, but at a much lower rate.
Just a thought, but it avoids some of the artificial impacts of some of the current systems, where one day you know nothing and the next you are an expert; and the situation where your country has been specializing in a certain doctrine from 1936 to 1944, and when your opponent gains it in 1945 they are all of a sudden just as good at it as you are.
