If making a pure rauchbier, then 50% plus of smoked malt is doable, but it will be really smoke forward...where you are actually making a smoked porter, not a rauchbier, then the 15-20% amount is probably fine. Could also make a 1 gallon test batch and if it's not enough, increase the smoked malt in the next batch.
This is the smoked robust porter recipe in Brewing Classic Styles (adjusted for my system):
Recipe Specifications
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Boil Size: 7.00 gal
Post Boil Volume: 6.00 gal
Batch Size (fermenter): 5.50 gal
Bottling Volume: 5.00 gal
Estimated OG: 1.067 SG
Estimated Color: 39.1 SRM
Estimated IBU: 34.6 IBUs
Brewhouse Efficiency: 64.10 %
Est Mash Efficiency: 67.1 %
Boil Time: 60 Minutes
Ingredients:
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Amt Name Type # %/IBU
8 lbs 8.0 oz Pale Malt, Maris Otter (3.0 SRM) Grain 1 54.0 %
3 lbs Smoked Malt (Weyermann) (2.0 SRM) Grain 2 19.0 %
1 lbs Caramel/Crystal Malt - 40L (40.0 SRM) Grain 3 6.3 %
1 lbs Caramel/Crystal Malt - 80L (80.0 SRM) Grain 4 6.3 %
1 lbs Munich Malt (9.0 SRM) Grain 5 6.3 %
12.0 oz Chocolate Malt (350.0 SRM) Grain 6 4.8 %
8.0 oz Black (Patent) Malt (500.0 SRM) Grain 7 3.2 %
1.25 oz Goldings, East Kent [5.00 %] - Boil 60.0 Hop 8 19.7 IBUs
0.75 oz Willamette [5.00 %] - Boil 30.0 min Hop 9 9.1 IBUs
0.75 oz Willamette [5.00 %] - Boil 15.0 min Hop 10 5.9 IBUs
0.40 oz Goldings, East Kent [5.00 %] - Steep/Whi Hop 11 0.0 IBUs
0.40 oz Willamette [5.00 %] - Steep/Whirlpool 0 Hop 12 0.0 IBUs
1.0 pkg California Ale (White Labs #WLP001) [35. Yeast 13 -
Mash Schedule: Single Infusion, Medium Body, No Mash Out
Total Grain Weight: 15 lbs 12.0 oz
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Name Description Step Temperat Step Time
Mash In Add 5.00 gal of water at 170.4 F 154.0 F 60 min
Sparge: Fly sparge with 3.89 gal water at 170.0 F
Notes:
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