My thoughts on it would be to test it using the whole workplace as the measure.
Lets use 10000 as the total staff in the department. You have a ballot and 5001 support it, then I think that an error of 1 paper would be reasonable and so on.
I am probably in the minority in the union movement, where I think the proposed legislation should be changed to workforce rather than membership. I believe that this will be a benefit to the union movement. There is no point delivering a weak mandate in the 60-70% area, let alone below 50%. As I have said before, make the right to strike and action short of a strike legal and protected, but only on a very high vote result, say 85% of the total workforce in the affected area.
This will force people to join unions and vote if they feel that there is a relevant issue, rather than sit on the backs of others.