There are + 44,000 zip codes in the US. They are arrange from east to west, and they are not sequential due to population grow.
Wouldn't it be easier to assign territories by State?
 
"UPDATE BKARCUST SET BKAR_TERRITORY = 'Number Uno' WHERE BKAR_STATE = 'FL';"
Where 'FL' is Florida and 'Number Uno' its territory.
 
If that does not work for you guys, what about this?
 
The first three zip digits are the code for the main sorting center, which distributes mail to the post offices (which are identified by the last two digits).
 
"UPDATE BKARCUST SET BKAR_TERRITORY = 'Number Uno'  WHERE LEFT(BKAR_ZIP,3) = '123';"
Where '123' is the sorting center and 'Number Uno' its territory.
 
Have fun  
 
Kelloggs
 
PD: you can use Evo's SQL Editor  
System Manager TAS-R
T7SQL.RWN