multiply a certain value to another from( other field) remain blank if no entry made

I would like to have a formula run the entire length of a column, but appear blank until info is entered. What I want is to have a formula that I can enter 1 number into this field and it does a calculation using this number and a number from another field. i.e I enter .25 in this field and it then multiplies that times 600 which is from another field. sorry if this is confusing and thanks for any help.