Hey Nedra! Since formulas in Airtable apply to every row, you can't modify the one client directly, but you can make a new field that can override your default value.
Create a new number field and call it "Days to Renew" or whatever you'd like and then modify your existing formula to be:
IF({Total Monthly Rental Fee w/Tax}>0,DATEADD({Staging Date},{Days to Renew}+1,'days'))This will make it so that whatever value you put in the Days to Renew field will be the number of days your date is pushed by. Now, let's add in another IF statement so that you can keep your default value of 45 days.
IF({Total Monthly Rental Fee w/Tax}>0,DATEADD({Staging Date},IF({Days to Renew} = BLANK(), 46, {Days to Renew}+1),'days'))That says, if Days to Renew is blank, add 45 days. If it's not blank, increase the date by the number listed.