Hi Arthur,
The "at isolation level" clause applies to the whole query and all the tables involved and so should come at the end of the query.
Individual tables can use different isolation levels within the query by using the holdlock, noholdlock and shared keywords after the table name. Some combinations of table level and query level locking make no logical sense though and will raise a warning or an error.
In your example you appear to just want a whole query to run at isolation level zero so you are correct to just put "at isolation 0" right at the end.
Cheers,
Andy Ashwood