Hi Grace, Don't make me say it: you gotta read the various saving the whites tutorials in the ER! Best as it says there is to get it right in the field; next best to recover the whites during conversion....
What are you using to convert?
IAC, I selected the whites with Select/Color Range and did a 100% Linear Burn on them (just as it says in the tutorials...) Usually about a 20% Linear Burn (use the Opacity Slider to adjust, just as it says in the tutorials...)