Sorky, sorry to hear your midwife wasn't very helpful.
FWIW, I had my nuchal scan done at the Fetal Medicine Centre and their "cut off" is 1:300 - i.e. they will offer/suggest diagnostic testing (CVS or amnio) to anyone whose risk is higher than that. I'd say it's still worth looking at what your age-alone risk would be - how old are you? For example, my risk based on age alone would have been 1:148, so I'd have probably been happy with a 1:256 result. At the end of the day though, whatever result you get, it's still only a risk assessment. Mine was 1:3000, but my baby could be that one. We decided against further testing but only because the risk of losing the baby was higher than the risk of him/her having a chromosomal abnormality. Had I been borderline, I'd already decided I'd go for CVS. This can be done earlier than amnio so less stressful waiting around.
Ultimately, though, only you can decide what to do. If the baby had Down's, would you keep it? If so, there's probably no point in opting for further testing. If no, then you might want to consider it . Sorry - no easy answers, are there?
I recommend going for a short run. It'll clear your head and enable you to work this through in your mind. Trust me - I solve all my problems and stresses by pounding them into the pavement.
Post here as much as you like - we're here to help (not that we can really be of much help in this case, I know ...)