To attempt to answer your question:
Crested budgies apparently are in possession of a very strange gene, which, although autosomal dominant, only acts dominant in certain cases. If you have a DF crested, it will be crested, but if you have a SF crested, although it still has the dominant gene, it only will show the gene 15-20% of the time. So you could have a bird that visually looks normal but actually is SF crested.
Apparently, the best way to breed for a crested with the best possibility is to take a known DF crested (either from two visually crested parents, or with very noticeable cresting) and breed it with a SF crested, or a normal, both of which should be the result of crested parents and both which are indistinguishable from one another as the gene appears to be lacking entirely if the SF crested does not express the visual trait, much like the normal offspring of a SF and SF crested, which could be a normal.
So for a breeding pair, you definitely could breed a normal with your hen, or a cock that is SF crested, but visually normal. (Basically any chick from a crested pair who doesn't show a visual crest, if you have one) They could have one gene, making them SF, or zero, making them genetically as well as visually normal, but unless you knew the exact genetics of the parents it would be almost impossible to tell.
Although you could breed two crested budgies together, by breeding it with a visual normal it cleans up the genetics a bit so it's not as cluttered, similar to the reason you don't breed two inos together.
However, both parings will yield crested budgies, DF x Normal and DF x SF. There's always high variability with such a weird gene
so it's likely you could get one or two crested chicks if you breed the hen with a normal cock. Unfortunately, these numbers are in no way guaranteed, sorry
You could end up with a lot of visual SF crested budgies, a lot of SF budgies who may or may not be crested, or a lot of budgies who might have a SF crested gene but there's no way to assure that when breeding with a normal cock.
Does that make sense?
Sorry for the confusion, let me know if I can clarify anything