I had the same issue with a similar process. For us, we had a form users can start and then submit on the website, when we tried to calculate submit %, we had the same issue as yours above (by day it was fine, but by service name we got over 100%). We discovered that this because people can come back to the submitted page more than once (either reloading it, or navigating away and then navigating back to it later).
Our solution was to make a segment that insists that users started the form before the submit happened. You could try making a segment to put around Page B that is something like "page A then within 1 pageview Page B", and add that segment to your metric.
Yes it would be a sequential segment. I would also suggest instead of just having "then" having it be within a certain amount of page views, depending on how your process works. For us, we had the form start and then within 2 page views a form submit (because some of the forms had more than one step).