I can't recommend a target Nitrogen is important when addressing different growth goals. In general, more nitrogen can support more growth.
I can use different fertilizers at any stage of development by adjusting application rates. When a fertilizer is high in nitrogen, I tend to use less of it, and when it is low in nitrogen, I use more of it. I tend to use a mix of fertilizers throughout the season to help trees get the micronutrients they need.
Michael Hagedorn has written some good posts on the topic that might help: