Pause and reconstruct only the missing step instead of restarting everything. If your setup is written clearly, you should be able to return to the line where the number came from and recompute it quickly.

This is mostly a note-management problem. Write intermediate results next to the expression that produced them and label what the number represents, especially when the calculation has several stages or units.

If you still cannot recover it, tell the interviewer exactly what you are doing: "I lost the intermediate volume figure, so I am going to recompute that line before continuing." That is much better than guessing and contaminating the rest of the math.

During practice, force yourself to write fewer but clearer lines. Good case math notes should let you restart from any step without recreating the entire page.