I think you should do a strand test on several sections of your head if you have different textures throughout. Doing it on one section that may take longer to get straight than others will give you overprocessed hair in the sections that do not require as much time.
For example, I take strands from the top, sides and back...and sometimes the nape area too. My sides do not need as much time as the rest of my hair, so I do that part last. This is a section that is approx. 3" by 3". The hair is finer there and the coils/curls are much smaller. I snip off hair at the roots from each section and tape them to pieces of cardboard and label which section it came from. I then go through the process of relaxing, smoothing, and neutralizing each section, while timing it. I then condition each section and air dry. Since I am texturized, these strand tests are very important to me, especially during retouches. I can't say enough about the necessity for doing these. It doesn't take long to do and can save you from having under or overprocessed hair. If you do not know which strength to use, strand tests will help with that as well.
