[You must [calculate ...]]
 [You should [go ...]]
Some grammars claim that "must calculate" and "should go" are verb groups, and thus are single grammatical units, or constituents, called the verbs.
But this isn't a very satisfactory analysis. It's better to say that for every verb there is a clause, and as there are two verbs there must be two clauses. For example,  consists of a matrix clause (the sentence as a whole in outer brackets), and a subordinate clause, "calculate ...", in inner brackets.
In other words, the sequences "must calculate" and "should go" each consist of two verbs, not just one. Thus, "must" and "should" are the matrix verbs and "calculate" and "go" are the subordinate verbs