when is an essential gene essential

[DrD]

I need to define a set of rules for when a gene can be assigned as 
essential. We are trying to knock out genes in Candida albicans using 
urablastoma (yeast suggestions would suffice though).
What range of frequency would you expect for non-essential or essential 
genes and what minimum set of experiments/criteria would you set before 
defining a gene as essential? What controls would you include to give 
greater confidence?
I am thinking along the lines of:
knowing the transformation frequency, the heterozygote would give x 
colonies of the correct structure per ug of DNA (screen by PCR, verify 
with Southerns)
again, knowing the transformation frequency, the homozygote would give x 
colonies of the correct structure per ug of DNA (screen by PCR, verify 
with Southerns) if the gene were non-essential
if the homozygote cannot be obtained then the frequency of the 2nd 
allele could be determined. An intact copy of the gene could be 
introduced, then the knockout formed. Or a conditional mutant produced.

any thoughts, particularly expected frequencies are greatly appreciated
Thanks in advance
David Haydon
when is an essential gene essential