There are numerous questions that come up for me. First, I don't know that paper by Miller et al., so I don't know what method it uses for estimating WTP.
Next, if you are using our ACBC software to calibrate the None utility per the threshold on the 5-pt scale, then it only changes the utility of the None parameter. The other utilities remain unchanged. So, you'll want to investigate what the Miller et al. WTP method does with respect to the None parameter.
Other questions come up in my mind such as data cleaning for respondents who are not consistent in their ACBC choices. Also, I wonder whether you have implemented monotonicity constraints to constrain price to be negative sloping. If you have too many "bad" respondents in the dataset, it can make WTP estimation not robust.
Everything I know about ACBC suggests that when done well, the WTP results should be robust. So, there are so many questions surrounding what you have done.
You may or may not know that we have built in an automatic WTP estimation routine into our ACBC software's simulator (the Lighthouse Studio Simulator).
You can read about our WTP methodology here: https://sawtoothsoftware.com/resources/technical-papers/estimating-willingness-to-pay-in-conjoint-analysis
And you can search the help in Lighthouse Studio for "Willingness to Pay" or "WTP".