I agree with Ryan that the intent is the most important thing. That being said, I think sanskrit is soooo beautiful and I personally try to go with it as accurately as I can, though my pronunciation isn't perfect. I try to feel the mantra, refer to sanskrit that I've heard and also get clues from the way that the roman characters are used to write the words (which is sometimes kinda diferent from how we use them in day to day english).
I'll have a crack at typing the Dharani in phonetics according to how I think it is pronounced.
Where theres a line above a letter its a point of emphasis so I'll write it in capitals, it can be explained as long two beats like 'a' as in 'calm' whereas a without the line above is short one beat like 'a' in 'america'
the 'c' in the name Cundi is used as we would if c and h were together as in the word
namaḥ: na-mah. the constonant 'h' is pronounced with the breath.
saptānāṃ: saptA-nAm nam sounds like 'sum'
samyak: sum-yuk (yuk as in duck)
koṭīnāṃ: ko-ti-nAm ko as in 'go' and nam as in 'calm'
tadyathā: tad-ya-thA: tad as in 'mud', thA: t and h are pronounced seperately, not together as in 'the', rather, 't' and 'h', then the final a is emphasised a bit as if there were two a's instead of one, kinda like 't-hA'
oṃ: I think this can be short ie 'om' as in 'on', or long as ommmm which sort of fades
cale: cha-lay, lay as in prey
cule: chu-lay, chu 'u' as in book,
cundī: chundi, 'u' as in book 'choon-di'
svāhā: the 'v' is pronounced as 'w' 'swA-hA'
.....so it might sound something like this:
na-mah saptA-nAm sum-yuk sum-boodd-ha ko-ti-nAm. tad-ya-thA oṃ. cha-lay chu-lay choon-di swA-hA
The Mahacundi Dharani is so beautiful, so shining and bright.
This is my best effort at writing it in phonetics according to my limited understand and my inner 'feel'I hope it's OK.