Such certainty , if based on evidence from the majority of other similar monuments might be understandable if optimistic , but considering that there is no reliable study that has shown any relation between Portal Tombs and the Thom paradigm alignments it's at the very least ,and trying to maintain a level of politeness , illogical .
There are a the huge amount of possible variables involving the arrangement of the stones and it is it's likely you could manage to find the required declinations among them , just as you could in your living room but that does not mean it is meaningful or intentional .
BTW declination of Deneb will be the same for it's rise and set ,it changes very slowly .