Unless you need to store the values for later use, I'd do it this way:
print "$1 ", sprintf("%5.2f\n",$3*0.4/100);
This Month: this11360this
Last Month: last5350last
Edited: Removed unnecessary extra colon in print (I had captured it already from the input) and inserted actual output generated - duh!
This Month: 45.44
Last Month: 21.40
I'd like to be able to assign to an luser