One thing that I noticed is that if I put a value in the "search envelope", hit the close button and open it again the size of the envelope will change.
Just a bug, I've had this issue too. If you want it to be exact, make your own cage and use xNormal! Otherwise set the bake size/quality low and do some test bakes until it looks good.
Not sure this is what you're looking for but you can create a custom envelope using the low poly.
Add your low poly to the "Target Mesh" section, add the cage (duplicated low poly and tweaked) to this tab as well, right click the cage file name and set "Use Selection as Envelope".
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One thing that I noticed is that if I put a value in the "search envelope", hit the close button and open it again the size of the envelope will change.
One more question, if I want to tweak the cage using maya I can't right? because of the bug
Add your low poly to the "Target Mesh" section, add the cage (duplicated low poly and tweaked) to this tab as well, right click the cage file name and set "Use Selection as Envelope".