Some like to make demarcation lines as to where it starts, but it's not really that easy, probably easier to measure hairline, frontal third, midsection, and then crown as approximate regions. The crown opens on all sides, from the frontal, sides and dropping down the back, some cultures almost have a double size when it thins on top and then drops down, almost potentially doubling the surface area.
A small crown can easily be almost a circle say 6x6, and for easy measuring, but for those in their maths comes the PI equation, this calculation is designed to give the values of a circle.
This makes it much harder to treat a crown, where to start, how many grafts to use, and where to place them because the probability is the surface area can increase easily by just a little thinning, for example, if the crown opened by 1cm more all the way around the surface area has dramatically altered.
This is why, and this goes for the frontal sides receding down also it is important to bite into the existing hair a little to ensure for future loss to some degree; rather than place only in the obviously thin area only for a year or two's time the thinning progress and being left with an island of hair.
The crown also because of design and surface can have an insatiable appetite or need for hair, the placement differs from the frontal areas so needs more to give the same appearance of thickness, and because of this, it could need a good 1/2 again compared to the frontal/mid. Generally, someone`s donor cannot give this (like mine now using beard and body) so a thinner look form a HT or lower density placed is common in the crown.
IF you start working in the crown first then we must be mindful not to "backload" the HT, using most of the resources available only in the crown and not enough to address frontal loss and being able to balance the look of a natural if thinning head of hair.
So, from the hairline back to a line drawn between the two temporal angles measures an area of roughly 50 cm2. The top of the head, from behind the frontal area to the front border of the crown may be about 150 cm2. The crown, as we pointed out can vary widely in size, but in a Class 6 or 7 can be as large as 175 cm2. Even if we transplant a minimal density (15 FU) to a fully bald crown (175 cm2) we have used roughly 2600 FU. If we transplant a higher density, 40 FU, then we have used 7000 grafts, this is more than the average person has available in their donor area and this is just in the crown.