docstr: Maximum number of bytes that can be added to characteristic
  while still being beneficial to merge it into another one.
code: def get_room(self):
      """Maximum number of bytes that can be added to characteristic
      while still being beneficial to merge it into another one."""
      count = len(self.items)
      return max(0, (self.overhead - 1) // count - self.width)
  
imports: from fontTools.misc.roundTools import noRound, otRound
  from fontTools.misc.intTools import bit_count
  from fontTools.ttLib.tables import otTables
  from fontTools.varLib.models import supportScalar
  from fontTools.varLib.builder import buildVarRegionList, buildVarStore, buildVarRegion, buildVarData
  from functools import partial
  from collections import defaultdict
  from heapq import heappush, heappop
label_desc: Use image node_id 4 for calling the _Encoding obj's underlying member method code with example usage: obj.get_room() and returns: max