There is a rule which I have learned to know and love by the name consecutio temporum, and it governs the tense of a conjunctive predicate in (many) subordinate clauses. All three Latin Grammars I have (Pekkanen, Streng, Pakarinen) indicate that if the governing clause is in the present tense and the subordinate clause refers to later events in active voice, one must use the active periphrastic conjugation. For example:
Nescio an saltaturus sis.
I don't know if you are going to dance.
Is it really necessary to use the periphrastic conjugation in such cases in classical Latin? Would it be possible to substitute it with a present conjunctive (saltaturus sis > saltes)? In particular, are there examples in known classical literature where the subordinate clause clearly refers to the (somewhat distant) future but a present active conjunctive form is used? Or are there reliable sources stating that this is indeed a hard rule? It is possible that there was a misunderstanding in the first Finnish Latin grammar and it was perpetuated through newer versions, so I want to be sure.
I know that the present tense can be used for things in the immediate future, so cases where one would clearly use the future tense in indicative are more convincing. Also, it is important that it's active, since the passive periphrastic conjugation is not used this way.