P.S.: As I like to check out things each version newly and especially after I have listen to Bret’s presentation again (If in doubt, Bret is right! Seriously.), I have put a short one minute clip together. Please explore the options with and without selecting, as Bret demonstrated it in the short time after minute 26.
Please check everything first on a copy, as I have not at all any idea about your scene nor any parts of it.
In the first half I set up the scene, hence the fast forward “style”, then I delete half of the weight, set it to absolute 0%, to simulate the missing other half, after that I copy, flip, and then merge. Note that anything that was selected (points) will be copied. The merge adds values! It might need some “rounds” to get used to it. I like small set ups and do such things until they are savvy. I do not recommend to start with complex characters directly.
My best results are given if I select what I like to mirror, then zero out the area and the area where I like to have it mirrored, replace, flip, merge.
https://www.amazon.com/clouddrive/share/EHTV0V8hFVBmYYJZu0U2BE5hmaf4kyRzvIwoxl14Pjw?ref_=cd_ph_share_link_copy
And with cleaning up the target area and how to get only a certain part of the weight into the mix:
https://www.amazon.com/clouddrive/share/ikx8Fx2xlRapzWjRCFG5UfHzS0l5yLNmibkYMB18XFf?ref_=cd_ph_share_link_copy
I edit it too fast, if you download, you will see that both sides at 0:35sec are weighted, please download it.
Refreshing the set up by disable/enable the Skin might help.
Again, perhaps I miss something, but why not using the Mirror Tool?
My best wishes for the project