NO. Adding 1 part cocoa solids needs to be divided by the total. 1 part solids/1 part solids does not have any meaning.
Chocolate: 0.3 parts (30%) solids and 0.7 (70%) parts other.
Example - add 0.5 parts solids: now 0.8 parts solids (0.3 + 0.5) plus 0.7 parts other = total 1.5 parts (0.3 + 0.7 + 0.5 = 1.0 choc + 0.5 solids = 0.8 solids + 0.7 other = almost 50/50 ).
0.8 parts solids/1.5 parts total = 53% solids.
Chocolate: 70% other as base - needs to be changed to 20% new mix. 70/20 = 3.5 parts total needed.
You already have 1 part chocolate: 3.5 parts total - 1 part chocolate = 2.5 parts of solids added.
0.3 parts solids (from chocolate) + 2.5 parts solids added = 2.8 parts solids 3.5 parts total (solids and other)
2.8 total solids/3.5 total mix = 0.8 = 80% solids