I wanted to ask about the letter (Dhaad) -I feel when I do the istitaala that the air goes back into my nose partly (as my tongue shuts off the makhraj from the front).

Question
Alhamdulillah I've always found your website very beneficial ma sha Allah, May Allah reward you for all the efforts put into it

I wanted to ask about the letter  ; (Dhaad) -I feel when I do the istitaala that the air goes back into my nose partly (as my tongue shuts off the makhraj from the front)

It's not exactly a nasal sound but is this a mistake? I notice it more if I'm reading in tahqeeq and so the timings are longer overall -I feel that some of the air goes into my nose...

Jazakumullah khayran

wa assalaam wa rahmatullahi wa barakatuh

Answer
Jazakum Allahu khairan.  May Allah accept it from us and grant you beneficial knowledge. 
The characteristic of is a mechanical moving of the tongue and it should not affect the sound.  The lengthening of the sound of the saakinah is due to the characteristic of rikhaawah and it should be the same length of any of the other saakin letters of the rikhaawah group. 
In general the sound should not go back through the nasal passage, but to check if it is affecting the sound in an incorrect way, close your nostrils while pronouncing the saakinah, and if the sound becomes muted, then it means there is a ghunnah, which is incorrect.  If however the sound remains the same, then there is no error insha' Allah as far as the nasal passage being used. 
The best way to make sure it is correct is to have a qualified sheikh or sheikhah listen to your recitation. 
Please also see:

http://www.abouttajweed.com/the-characteristics-of-the-arabic-letters/characteristics-of-the-letters-part-8-characteristics-without-opposit.html

Wa iyyaakum.  Wa assalaam alaikum wa rahmatullahi wa barakatuh.