We actually have weight locking as a proposed enhancement here:
https://github.com/EsotericSoftware/spine-editor/issues/338
I've added your +1!
You should avoid having dense meshes in the first place, about this here is a nice read about good mesh structures:
Blog: Mesh creation tips: vertex placement
Blog: A taxonomy of meshes
Blog: Mesh weight workflows
But of course, where there's a will there's a way. If you just have to apply 60% of a bone influence to several vertices, then select all of the vertices, select the bone in question, and set the weight to 60%. It will be applied to all of the vertices.
I presume that's not really the problem though, is it? you likely need to get some other weight to a certain specific amount, and then ensure this specific bone is still at 60% after changing it. You could tweak the weights and then set the amount back to 60% and that would work, but it would wreck the previous weight.
So in case you need to have a specific set amount for a bone weight, and a specific weight also for another bone on the same vertex, here's how I do it:
Let's say that I want my weights to be 20% one bone, 20% of a second bone, and 60% of this third bone.
I'll set 100% of the weight on the first bone, then 50% on the second bone: this brings the percentage of both to 50%. Then I'll apply 60% of the third bone weight to the vertex. The setup is achieved.
This works because whatever percentage I apply last will eat up the amount I set and proportionally resize the others.
Since you have a 60% weight fixed on the third bone, that leaves 40% max on the other bones to spare.
Say you want to achieve the first bone set to 10% and the second one set to 30%: that's basically 1/4 for the first, and 3/4 for the second bone.
You'll set the first to 100% (to have a clean slate) then the second to 75%, then apply 60% of the weight to follow the third bone. Setup achieved again!
Whenever you want to recreate exact weights, always note down the percentage of the weight you remove, and the new weights without it, then set them again in reverse on the new vertex.
As Nate mentioned, applying specific weights is very easy also using the replace mode in the weights view: Weights view - Spine User Guide: Brushes
or using direct as I mentioned above.
If you concern is purely to see the result of the whole thing while moving, then my recommendation is to consider the approach of first making every piece move at their greatest range, and only later apply a global 3D effect, exactly to avoid having to go back and forth this way.
If you'd like to discuss what you want to achieve with the setup you're creating, I'd be happy to give more detailed advice (:
