I have no idea IF that ear comes with a calibrated plot but I am pretty sure sticking a calibrated omnidirectional mic inside a tube (the earcanal) will also influence or at least alter the FR the mic receives, aside from what the Pinna and canal does.

I mean... I don't think if you mount an omnidirectional mic on a surface or in a tube the reference plot will be of much value as it was made in free-air. It won't be flat with a few dB peak around 16kHz any more is what I am getting at.

IF you intend to make absolute accurate measurements with it, it would have to be calibrated with a reference headphone that already is calibrated/known.

As a quick and dirty 'lets at least get a bit closer' practical method would be to measure 1 (preferably more) 'known' headphones.
Of course there are variances between them as well so unless you have the actual one that has been measured on a real calibrated rig (that is properly 'compensated') you will have to allow some errors.

The next 'hurdle' would be to know WHICH 'correction' the calibrated head measurer did use to generate the plots ?
Did they use the Olive-Welti compensation, farfield or nearfield compensation, something in between or custom ?

If you do not have access to a calibrated headphone a cheap method would be to use a 'well known' headphoneThe HD600/HD650 come to mind as excellent 'reference' phones for this.
You would have to allow for product variances though or perhaps lend one or more 'known' phones.

The next question would be WHO makes the best 'reference' measurements that come closest to reality ?
The manufacturer ? Tyll ? Purrin ? Ultrabike ? Rin Choi ? Golden ears ? Stereophile ? about.com ? review.com ? any of the Russian, Korean or Japanese sites ? or even others ?
Did they use the Olive-Welti compensation, farfield or nearfield compensation, something in between or custom or just publish raw plots ? Does one know which compensation was used on any of those sites ?

In any case you will be able to compare your own measurements against those of your own and see what the changes are you actually made when modding.
How close your plot will turn out to be to what's really happening will thus depend on a shitload of factors aside from the (invalid ?) calibrated mic and needed compensation for THAT specific 'ear'.

Lots of research and overlaying plots (by stretching plots so they have the exact same scale !) may teach you what you are looking for.

Thanks for the reply. So only way to do is measure a known headphone and compensate the difference.
So it can be done without that artificial ear also.

yes, it can be done without an artificial ear BUT mounting a mic on a hard (echoing) surface will create other problems that need to be compensated as well. Care must be taken to mimic thin skin on bone.
Also the space the ear takes up as well as the 'shielding' of soundwaves from the rear also has some influence on the plots you get that way.

To make things worse... for instance when you measure 2 different headphones on the same 'flatbed' rig and add a Pinna (so without the ear canal) the plots you get may differ yet again.

A speaker sound comes from your in front of you and the ears are 'trained' to interpret that.
A headphone sound comes from the side and the pinna works VERY different from that angle as from the front.
When you listen to your speakers from a few meters in front of you and then move them much closer and on the far right and left the same speakers will sound VERY different.

This is why it is hard to measure headphones 'correctly' and the 'needed' compensation is debated quite often.
Who is right, who is wrong or perhaps who is more right or wrong ?

If the plots 'show' what you are hearing you will be close.
I would dare to say that trained ears are almost a must if you want to 'calibrate' that way (by ear).

Now I have lot of iem for which graphs are available.
But for headphones I have just  vintage orthodynamic headphones which I need to modify.

For in ears you can get away with a specified length of tube but personally think the artificial ear is more suited.
As Marvin (Purrin, Marvey) also clearly states measurements done above 9kHz are not very reliable as the compensation you need will differ yet again and the insertion depth as well as getting a good seal may not be that easy to achieve.

Here is example of zebrano wood T50rp I built.
http://cdn.head-fi.org/9/99/99f28a47_t50rp-wood.jpeg
http://cdn.head-fi.org/4/4f/4f320076_t50rp-wood-wf.jpeg

It is hard to evaluate the plot if it isn't clear how it was created.

I think I need to build a portable measuring rig and take it to meet to measure some known cans .

http://cdn.head-fi.org/f/f6/f6f1a34f_HDF0024_9744CC_blog.jpeg

I wonder how he places headphones on the kit.