I seriously doubt that removing 20% will be any problem for the tree. The one thing that could be a problem is the wound left by removing a limb that big. Hopefully, you cut just outside the branch collar. Don't use any type of sealer on the wound, but you might look into liquid copper fungicide previously mentioned in this forum. On the other hand, you mentioned that it had "quite a bit of deadwood". If it were me, I think I'd just pat myself on the back for finally getting around to pruning it, and concentrate on my new garden.

I'm of the opinion that no one should be 'held hostage' on their own property over a tree, especially one that doesn't sound all that gorgeous in the first place.

A wonderful compromise would be to select a site on your property where you can plant a new tree. Put it someplace where it won't be a problem for future owners, as the tree grows large.

In a couple of days I will post a new topic about the use of liquid copper fungicide to prevent rot from entering large pruning cuts and large wounds to the trunks of trees. I have posted about this many times before and searching this site and scrolling through the topics that appear will turn up what I have said in the past.

Of course if the tree already has a lit of dead wood--possibly large dead branches that may already provide entry points for rot, the benefits of any treatment may be limited.

Anyway. I will post under a new topic with the appropriate search words in the title, what I know about the use of liquid copper fungicide as soon as I get the time.

--Spruce