Correct RX_PACC adjustment

Hello. I have a question about the correctness of calculation RX and FP Powers.
One of the input variables to logarithm is RXPACC field. And sometimes it needs an specific adjustment.
So my question is what should I do if when I need to substract the adjustment from the input variable N(RXPACC) before send it to logarithm but adjustment is greater than the value of this variable RXPACC itself? There will be an overflow. Is that possible situation?
The only solution I found is to write 1 at N if RXPACC <= ADJUSTMENT.
But I’m not sure it’s correct. Thank you.