Cross cultural dialog belief

By using the cross cultural dialog belief (1 science for every four believers) and sending out a gazillion missionaries to the neighbors on a continent in which I was the only civ with a religion, I was able to generate 25 percent of my science from those believers, in addition to my campuses and such. Is that the max of what you can do with that belief or have you been able to get more than that? Trying to decide whether to do that again on the next game as managing the missionaries was kind of tedious.

For reference, this was emperor, playing as Gaul, Gathering Storm.

Also, when you have your first Apostle and you're wondering whether it's worth using him to get an additional belief, is there an easy way to know what's still available? I keep getting confused on that.