Hi,
Yes, tapering is defined in worlds units because the full length of the branch is not known while ForestIvy is generated.
And the branch width (which depends of tapering) is necessary for some processes, as the leaves placement.
To use a percentage, it would require to build ForestIvy in two stages: first the branches, and next other processes which depend of the branches.
This is more complex and less efficient, specially for the multithreading implementation.