No matter what you do, you will eventually have finished compost. Too much manure and the pile will get smelly and the process will slow down. Too much dry plant matter and the pile will smell fine, but the process will be even slower. The science of composting is finding exactly the right balance to produce finished compost in the minimum amount of time and to get things hot enough to kill weed seeds. But, if you have more important things to do than compost experiments, just put in roughly equal portions and you will get finished compost in 6 months to a year. It might have some viable weed seeds in it if you're too far off the mix, but that's not the end of the world.