Calculating percentage from eneterd values within a table/form

Within my database I have a table where I will enter the fresh weight of an
object in one field and the thawed weight of the object in another field.
I then wish access to calculate the percentage weight loss by :
(Fresh Weight-Thawed Weight)/Fresh Weight*100

I have attempted to do this as a default value of a field during the design
of a table, but as data has not been entered into the weight fields access
gets confused and won't allow this.

Is there a way to do this without creating a query or doing it within a form?
If I have to create a query or do it within the form, is there a way to link
back the calculated value so that it is displayed in the original table?

I am a novice at Access and have only been on an introductory course to this
programand so have limited knowledge. All mention of macros and code swoops
over my head!