"Calculated" fields in your database program can draw data only from existing fields defined as "Numeric" or "Currency" fields in your database file. You cannot use a "Calculated" field as a component of a formula for another "Calculated" field. This is due to the inherent nature of the database file set for saved information.
This does not mean that you cannot use Calculations from other Calculations - you just need to re-copy the formula from your initial Calculation into the second Calculation's formula using Windows' copy and paste function and enclose it with parentheses "(" and ")" in the formula bar.
For example, you might have a Calculation in your database to turn a amount in a field called "Number1" into a decimal to be used as a percentage that will be displayed in the Calculated field called "Percent1", like this:
<<Number1>>/100
This Calculation is legal and would simply divide a figure in the field "Number1" by 100, thus turning a numeric entry like "12" into "0.12" in the resulting "Percent1" Calculated field.
If you then wanted to use this figure to multiply by an amount (like a "Price1" field, in order to figure a resulting tax), you could not do the following Calculation:
<<Price1>>*<<Percent1>>
This would not be allowed because "Percent1" is a Calculated field.
Instead, simply copy and paste the text that made up the formula for the initial Calculation into the new formula in the appropriate place, like this:
<<Price1>>*(<<Number1>>/100)
This would net you the same result, and this formula is allowed.