The next page in the Red Book specifies an armouring scheme for the Ger/CBL/1915 Mod.A design

I have taken the Ger/CBL/1915 design and applied the armouring scheme specified by a cross sectional drawing in the Red Book.

The features are a 4in outwardly angled main belt, 12ft high, a 8-ft high upper belt, 3in thick, and a 2in upper deck. There is no torpedo bulkhead, although the hull is subdivided and has the "sandwich" layer at each side. I was able to retain the 40-knot speed, although the displacement grew. Still, the standard displacement is 19,793 tons, which is not too great. The draft grew to 25.5ft, as well. This was all to accommodate a greater armour protection, although the length was shortened. Presumably increasing the deck armour to 2in from 1in was the main culprit. This is the Springsharp report:

Ger/CBL/1915 Mod.A High Speed  Red Book, Germany Light Battlecruiser laid down 1915 (Engine 1950)

Displacement:
19,159 t light; 19,793 t standard; 20,900 t normal; 21,785 t full load

Dimensions: Length overall / water x beam x draught
802.24 ft / 800.00 ft x 80.00 ft x 25.50 ft (normal load)
244.52 m / 243.84 m x 24.38 m  x 7.77 m

Armament:
     2 - 12.00" / 305 mm guns (1x2 guns), 864.00lbs / 391.90kg shells, 1915 Model
  Breech loading guns in a turret (on a barbette)
  on centreline forward
     12 - 6.00" / 152 mm guns in single mounts, 108.00lbs / 48.99kg shells, 1915 Model
  Quick firing guns in deck mounts with hoists
  on side, evenly spread
Weight of broadside 3,024 lbs / 1,372 kg
Shells per gun, main battery: 120

Armour:
  - Belts:  Width (max) Length (avg)  Height (avg)
Main: 4.00" / 102 mm 410.00 ft / 124.97 m 12.00 ft / 3.66 m
Ends: Unarmoured
Upper: 3.00" / 76 mm 410.00 ft / 124.97 m 8.00 ft / 2.44 m
  Main Belt covers 79 % of normal length
  Main belt does not fully cover magazines and engineering spaces

  - Gun armour: Face (max) Other gunhouse (avg) Barbette/hoist (max)
Main: 4.00" / 102 mm 3.00" / 76 mm  4.00" / 102 mm
2nd: 2.00" / 51 mm 2.00" / 51 mm  3.00" / 76 mm

  - Armour deck: 2.00" / 51 mm, Conning tower: 4.00" / 102 mm

Machinery:
Oil fired boilers, steam turbines,
Geared drive, 4 shafts, 282,797 shp / 210,967 Kw = 40.00 kts
Range 6,200nm at 15.00 kts
Bunker at max displacement = 1,992 tons

Complement:
868 - 1,129

Cost:
£2.815 million / $11.260 million

Distribution of weights at normal displacement:
Armament: 378 tons, 1.8 %
Armour: 3,177 tons, 15.2 %
   - Belts: 1,274 tons, 6.1 %
   - Torpedo bulkhead: 0 tons, 0.0 %
   - Armament: 330 tons, 1.6 %
   - Armour Deck: 1,508 tons, 7.2 %
   - Conning Tower: 65 tons, 0.3 %
Machinery: 6,765 tons, 32.4 %
Hull, fittings & equipment: 8,839 tons, 42.3 %
Fuel, ammunition & stores: 1,741 tons, 8.3 %
Miscellaneous weights: 0 tons, 0.0 %

Overall survivability and seakeeping ability:
Survivability (Non-critical penetrating hits needed to sink ship):
  20,174 lbs / 9,151 Kg = 23.3 x 12.0 " / 305 mm shells or 1.8 torpedoes
Stability (Unstable if below 1.00): 1.30
Metacentric height 5.8 ft / 1.8 m
Roll period: 14.0 seconds
Steadiness - As gun platform (Average = 50 %): 51 %
  - Recoil effect (Restricted arc if above 1.00): 0.26
Seaboat quality  (Average = 1.00): 1.02

Hull form characteristics:
Hull has rise forward of midbreak
Block coefficient: 0.448
Length to Beam Ratio: 10.00 : 1
'Natural speed' for length: 28.28 kts
Power going to wave formation at top speed: 58 %
Trim (Max stability = 0, Max steadiness = 100): 50
Bow angle (Positive = bow angles forward): 4.00 degrees
Stern overhang: -2.00 ft / -0.61 m
Freeboard (% = measuring location as a percentage of overall length):
   - Stem:  32.00 ft / 9.75 m
   - Forecastle (20 %): 27.00 ft / 8.23 m
   - Mid (68 %):  27.00 ft / 8.23 m (19.00 ft / 5.79 m aft of break)
   - Quarterdeck (15 %): 19.00 ft / 5.79 m
   - Stern:  19.00 ft / 5.79 m
   - Average freeboard: 24.84 ft / 7.57 m

Ship space, strength and comments:
Space - Hull below water (magazines/engines, low = better): 118.9 %
 - Above water (accommodation/working, high = better): 222.8 %
Waterplane Area: 40,765 Square feet or 3,787 Square metres
Displacement factor (Displacement / loading): 106 %
Structure weight / hull surface area: 138 lbs/sq ft or 672 Kg/sq metre
Hull strength (Relative):
 - Cross-sectional: 0.97
 - Longitudinal: 1.34
 - Overall: 1.00
Hull space for machinery, storage, compartmentation is cramped
Room for accommodation and workspaces is excellent