You need to have more copper between the pex and where u are sweating. Either all sweating is done before the crimping or you must have something like 18" + of copper.
I'm sure the pros will know more specific numbers, but that's what I did when I ran mine. Just keep the pex as far a distance away from where you arw sweating.
I know they sell copper stub outs for sink stops that are nothing more than a little L pex crimp and then turn out the wall. I would imagine Unless you are putting a compression stop on there then there is very little to sweat to.