column label

Ok I have a query in a program that looks something like this

select
reference_number as foo,
'(10,10)'::point <-> point(x,y) as distance
from imagemap
where
index_number = '2322' and
block_number = '140'
order by distance
limit 1;

I wanted to add a clause in where 'and distance < 30'
but according to the postgres manual it says I cannot do that for WHERE or
HAVING clauses, instead use the real name.

However dropping the 'as distance' from my query gives me a column name of
'?column?' which happens to not be worth a dang thing.

How can it be done?

(I suddenly have a feeling its going to require a subselect)

Ryan