I want to compare future perfect active indicative (f.p.a.i.) and perfect active conjunctive (p.a.c.). They look identical, apart from first person singular (cogitavero ≠ cogitaverim). But is there a difference in the length of the last -i-? It has to be short in third person singular and plural due to its position, but the remaining three persons (cogitaveris, cogitaverimus, cogitaveritis) are less clear to me.
What is the length of the -i- in these forms for future perfect and perfect conjunctive? Are the lengths different? I was taught that they are all short, but I have seen some sources mark the conjunctive long and the future perfect short, and I think I have also seen both marked long.
References to reputable grammars are welcome, but arguments about how we know the lengths would be most useful. My experience is that grammars disagree — although some may be in the minority — and it is not easy to tell what to trust.