Sorry for not picking up on this post before, because measuring nystagmus is an interesting -- and potentially controversial -- topic. Evidence for this came at the UK Nystagmus Network's 2009 international research workshop. The 30 or so delegates had some lively debates about how you measure nystagmus.
The simple answer to your question is No, there is no standard worldwide way of measuring the effects of nystagmus. There are some standards for measuring visual acuity (how far you can see). But nystagmus is dynamic and complex. So, simply reading letters on a chart at a set distance does not tell us a lot about our vision. For example, it does not take account of:
the null point
the time needed to see
the effects of stress, fatigue, etc
how does our vision cope with a moving image
Severel researchers around the world are looking at ways of measring the impact of nystagmus, but we probably have a long way to go yet before we see some global standards.
On a more positive note, the equipment to measure eye movements has improved considerably in recent years. It's much less intrusive and cumbersome. Unfortunately, not many hospitals either have or use the latest equipment for recording nystagmus eye movements, despite the potential benefits in terms of diagnosis and improving our understanding of the condition.