It really depends on you. In my experience when I try to stay up for like fifteen minutes or something I actually can't fall back asleep! I think it's actually a good idea to write down your dreams and then just go back to sleep and do a MILD as you do. But it all depends on you, if you feel like staying up longer will help you focus your intent more then go for it! The way you described is actually a way some of the people on here do it. It's short,sweet,and gets you focused on your intent without loosing a lot of sleep or anything.

One way I do a WBTB is I have a watch alarm that wakes me up like an hour before I actually wake up and carry on with my day. My alarm shuts off after about 20 seconds and I can fall asleep pretty fast. When I do I just really focus on having a lucid dream.

Just do whatever you feel will get you focused and wont make you loose a lot of sleep. Good luck and happy dreaming!