A quick Pythonista script (with UI elements) to calculate the percentage complete on a book with irregular page numbering.

BookPctCalc.py

import ui
import console


def calcPercent(total, start, current):
    pct = (current - start + 1) / (total - start + 1)
    return round(pct * 100, 1)


def buttonClick(sender):
    total = 0
    start = 1
    current = 0
    for ctl in v.subviews:
        if ctl.name == 'tfTotalPages':
            total = int(ctl.text)
        if ctl.name == 'tfFirstPage':
            start = int(ctl.text)
        if ctl.name == 'tfCurrentPage':
            current = int(ctl.text)
    test = calcPercent(total, start, current)
    console.alert(str(test) + '%')


v = ui.load_view()
v.present('sheet')

BookPctCalc.pyui

[
  {
    "nodes" : [
      {
        "nodes" : [

        ],
        "frame" : "{{6, 6}, {150, 31}}",
        "class" : "Label",
        "attributes" : {
          "font_size" : 18,
          "frame" : "{{45, 104}, {150, 32}}",
          "uuid" : "6DEE7310-0FAA-42D2-A1CD-761AB89408EE",
          "class" : "Label",
          "alignment" : "left",
          "text" : "Total Pages",
          "name" : "label1",
          "font_name" : "<System>"
        },
        "selected" : false
      },
      {
        "nodes" : [

        ],
        "frame" : "{{6, 45}, {200, 32}}",
        "class" : "TextField",
        "attributes" : {
          "font_size" : 17,
          "frame" : "{{20, 104}, {200, 32}}",
          "spellchecking_type" : "default",
          "class" : "TextField",
          "uuid" : "299139CB-B076-4467-A2A8-FD6D7C8ECCDE",
          "alignment" : "left",
          "autocorrection_type" : "default",
          "name" : "tfTotalPages",
          "font_name" : "<System>"
        },
        "selected" : false
      },
      {
        "nodes" : [

        ],
        "frame" : "{{6, 165}, {150, 32}}",
        "class" : "Label",
        "attributes" : {
          "font_size" : 18,
          "frame" : "{{45, 104}, {150, 32}}",
          "uuid" : "BC6DD854-502B-4EE9-B75F-BEA13A9258FD",
          "class" : "Label",
          "alignment" : "left",
          "text" : "Current Page",
          "name" : "label2",
          "font_name" : "<System>"
        },
        "selected" : false
      },
      {
        "nodes" : [

        ],
        "frame" : "{{6, 202}, {200, 32}}",
        "class" : "TextField",
        "attributes" : {
          "font_size" : 17,
          "frame" : "{{20, 104}, {200, 32}}",
          "spellchecking_type" : "default",
          "class" : "TextField",
          "uuid" : "D5A9508D-DD47-4574-8C3B-538D8908E6AA",
          "alignment" : "left",
          "autocorrection_type" : "default",
          "name" : "tfCurrentPage",
          "font_name" : "<System>"
        },
        "selected" : false
      },
      {
        "nodes" : [

        ],
        "frame" : "{{6, 85}, {150, 32}}",
        "class" : "Label",
        "attributes" : {
          "font_size" : 18,
          "frame" : "{{45, 104}, {150, 32}}",
          "uuid" : "CF04F9F7-802F-4C2D-B9C1-B3A5DEF8AF48",
          "class" : "Label",
          "alignment" : "left",
          "text" : "First Page",
          "name" : "label3",
          "font_name" : "<System>"
        },
        "selected" : false
      },
      {
        "nodes" : [

        ],
        "frame" : "{{6, 125}, {200, 32}}",
        "class" : "TextField",
        "attributes" : {
          "name" : "tfFirstPage",
          "frame" : "{{20, 104}, {200, 32}}",
          "spellchecking_type" : "default",
          "class" : "TextField",
          "uuid" : "A6B04FE7-90CC-4AB5-94D2-3DF4E3834513",
          "alignment" : "left",
          "text" : "1",
          "autocorrection_type" : "default",
          "font_size" : 17,
          "font_name" : "<System>"
        },
        "selected" : false
      },
      {
        "nodes" : [

        ],
        "frame" : "{{6, 242}, {164, 32}}",
        "class" : "Button",
        "attributes" : {
          "action" : "buttonClick",
          "border_width" : 1,
          "frame" : "{{80, 132}, {80, 32}}",
          "title" : "Calculate Percentage",
          "uuid" : "AA6D7021-B72E-4516-85DB-B019839D08FA",
          "class" : "Button",
          "corner_radius" : 2,
          "name" : "button1",
          "font_size" : 15
        },
        "selected" : true
      }
    ],
    "frame" : "{{0, 0}, {240, 357}}",
    "class" : "View",
    "attributes" : {
      "enabled" : true,
      "background_color" : "RGBA(1.000000,1.000000,1.000000,1.000000)",
      "tint_color" : "RGBA(0.000000,0.478000,1.000000,1.000000)",
      "border_color" : "RGBA(0.000000,0.000000,0.000000,1.000000)",
      "flex" : ""
    },
    "selected" : false
  }
]