The vowel sound you need for the nasalized vowel is given to you by the phonetic symbols.
- ɛ̃ (pain, brin, …) ← ɛ (brève, bête, …)
- ɑ̃ (entre, antre, …) ← ɑ (pâte, tâche, …)
- œ̃ (brun, jeun,… ) ← œ (heure, œuf, …)
- ɔ̃ (pont, pompe…) ← ɔ (pomme, motte, …)
Little difference is made between ɑ and a in many regions, so your suggestion of a nasalized version of the vowel a from matin would be closer to ɑ̃ than to ɛ̃, but ã is not a phoneme in French.
Parisian accent merges ɛ̃ (unrounded) and œ̃ (rounded) into a middle æ̃; it’s for them actually impossible to distinguish brin from brun. (You can find the associated vowel æ on the vowel chart, but it should really be interpreted in this case as an open-mid that is exactly between ɛ̃ and œ̃). So you will be understood even if you don’t make a distinction between those two, and I believe this is what your question was about.
Since ɛ and æ are English vowels, I’d say that for an English speaker ɛ̃ and æ̃ are the most easily accessible. So:
- if you want to sound Parisian: you can use [æ̃] for both /ɛ̃/ and /œ̃/.
- but if you are a respectable person: use [ɛ̃] for /ɛ̃/ (the one in matin), and you can use [æ̃] as a substitute for [œ̃], which makes them distinct. This way, when you’ll learn the more precise [œ̃] sound later, you will already know which words use which sound.
There is so many difference in the way to pronounce vowels…
I am not myself sure to pronounce matin in the same way at work or at home. What I am sure on the contrary, is that I will use the same phonemes for matin, cinq and calin.
I have counted 9 possible confusions: matheux, matou, maton and with conjugates of the verb mater (matant, mater, matait,…). Nothing that looks like a nasal central mid-vowel. So you can use any sound as long as it is nasal and non clearly articulated, something between in* (of brin) and un (of brun).
As a swiss speaker, I don’t think I’ll even take this as a regionalism differentiation, we have much more clear markers.