Clarification on the articulation point of the seen, zay, and Saad

Assalamu Alaikum,

I have noticed that the Makharij of seen,zaa' are different in different books. Recently I got hold of some picture prepared by Dr. Ayman Rushdi in which the picture of seen, zaa' is showing the tip of the tongue touching the plate of the lower incisors. I have personally found that this is more practical to apply. Someone also informed me that Dr. Ayman Rushdi has explained this is the correct way. Could you please clarify?
Wa alaikum assalaam wa rahmatullahi wa barakatuh.
 In the book about Tajweed called "Hilyat-ul Tilaawah which is written by Ash-Sheikhah Dr. Rehab Shaqaqi and reviewed and edited by his eminence Sheikh Dr. Ayman Rushdi Swayd, the makhraj  (articulation point) of the  are explained as follows:
It is articulated by placing the tip of tongue on the plates of the lower two incisors so that the sound is emitted from between the upper and lower incisors.
The tongue is on the top of the plate of the lower incisors, not in the middle or lower area, also please note, the pressure on the tongue when articulating these letters is very minimal.
Imam Ibn Al-Jazaree stated in his poem on tajweed that the “whistle” letters were from between the upper and lower incisors.  We can see then that the tip is placed on the upper part of the plate of the lower incisor, but the sound itself is emitted from between the upper and lower incisors. 
We plan to update the site soon to carry the more accurate definition, insha’ Allah.
Wa assalaam alaikum .