Can I have a formula but allow data entry in same cell?

Here is what I am looking to do.

Basically I have column b,c and d. Column b is manually entered with no formula

Column C Formula : =D19/(B11*B15)

Column D Formula : =B11*B15*B19*C19

The D19,C19 and B19 are in the columns so they progress up by one row until all three are at 36 (B36,C36,D36). The B11 and B15 are sums that are determined higher up on the spreadsheet.

As you can see column c and d rely on being able to enter data into them for either formula to work. however traditionally that erases the formula rendering my sheet useless unless only used as a one time operation which is not what I want. So can I protect the base formula while allowing the person to enter data into that cell? The end result will be the same number anyways. or Do I need to add two additional columns one for results one for entering?

Its basically kind of an either or situation. you either know column d and need to know column c or you know it vice versa.

Thanks.

In addition.

How can I copy and paste a formula that will continue to count up but only certain rows/columns.

Example : =B11*B15*B19*C19 and have it only count up like so =B11*B15*B20*C20 having it only count the last two figures.

Thank you.