Custom calculation getting incorrect results with blank fields

We have the following custom calculation script:

event.value = ( this.getField("Food Minimum").value + this.getField("Lunch Minimum").value + this.getField("Service Charge").value ) * ( this.getField("Food Service Tax %").value );

The problem occurs when Food minimum or Lunch Minimum are blank (no data), the calculation multiplies the value of the non-blank field, circumstantially. 

if Lunch Min is blank, and you enter a value of $1 in Food Min, it will calculate as $10 (one dec place)

if Lunch Min is blank, and you enter a value of $10 in Food Min, it will calculate as $100 (one dec place)

if Lunch Min is blank, and you enter a value of $100 in Food Min, it will calculate as $10000 (2 dec places)

if Lunch Min is blank, and you enter a value of $1000 in Food Min, it will calculate as $1,000,000 (3 places)

..up to a value of 4166 ..  enter value 4167 and the calculation will now go to 4 decimal places (41,670,000)

*if you reverse and leave Food Min blank instead, then Lunch Min seems to calculate x 10 with any value (one dec place shift)

any explanation or guidance would be great.  The answer is to make sure there's a "0.00" value rather than a blank field and then it calculates properly.

thanks