If you need to caulk a big gap, larger than 1/4", before you put any caulk in the joint get some foam backer rod and put it in there first. The foam backer rod will fill up a lot of the gap so that you need a lot less caulk. And since you use less caulk, there's less chance of the caulk shrinking or cracking. You can get foam backer rod at any hardware store, and it's really cheap. So it's a good investment so that you only have to do the caulking job once.