Welcome to HCM.
The thickness of the endometrium is not mentioned.
we classify endometrium based on appearance of endometrium on ultrasound. If we can clearly see triple line endometrium with a halo surrounding it, then it is proliferative phase, if it is uniformly echogenic with loss of triple line and thick, then it is luteal phase.
All said and done there could be interpretation error if date of last menstrual period is not taken into consideration.
More over if the periods are delayed then 16th day may not be luteal phase.
What I have mentioned is what is seen usually. There are many variations in the endometrial appearance depending on different medical conditions.
Nice to know, you have a fairly good idea of the subject.
Wish you all the best.