It seems to me that the new moon ought to be calculated according to the actual time of the conjunction. One reason I have already mentioned comes from Sanhedrin page 10 that several amoraim say that the new moon is not dependent on the court. this goes along with r the opinion of R. Elazar Ben Azariah that 'If the court does not sanctify it on time , they sanctify it from heaven." But a further reason come from the fact that a person that murders in front on a court is killed because דלא תהיה שמיעה גדולה מראיה that is to say certain kinds of things do not depend on the court knowing them through hearing. if they know the facts from a more accurate source of information e.g if they saw the events themselves, that is even better than hearing. There in Sanhedrin 78a where Tosphot brings this, it only says that seeing is better than hearing. But to me it seems the same idea applies. You can see this from the מלחמות of the Ramban where the Ramban brings this idea about Kidush Hachodesh specifically. [See Rav Shach's Avi Ezri הלכות סנהדרין פרק י''ד ה''א where Rav Shach brings these sourses.]
