20g of amarillo as a bittering amount is a guesstimation but one that assumes a full volume boil. If you are doing less than a full volume boil you'll need to make adjustments for that.
Ah ok, thanks, I hadn't realised that the gravity of the wort during boil made a difference to the IBU's added by the hops. I think you've helped me understand the "Hop Concentration Factor" on IanP's "Kit & Extract Beer Designer" spreadsheet...I had always wondered what the 'boil volume' and HCF were about.
It looks like if I put my boil volume in as 5 litres (after steeping my grain in a litre of water as you suggested), that I have to add roughly 500gm of LME to bring it up to a 1.040 gravity. Then I need to adjust the hop additions slightly to get to an IBU of 31. Here's the adjusted method, I think this addresses each of your concerns.
Method:
Boil 7 litres of water in my pot, then add it to the fermenter to cool.
Heat 1 litre of water to 65 degrees
Lower the flame so it keeps heating, but slowly
Add the 250gm cracked crystal malt grain in a grain bag, start timer
When heat approaches 75, turn off the flame
Dunk/swirl the grain bag every now and then, make sure temp doesn't drop below 65
While steeping, re-hydrate the yeast
When timer reaches 30 minutes, remove grain bag and allow to drain. Rinse grain back into pot with 1 litre of ~70 degree water
Add ~3 litres of boiling water from kettle to make 5 litres total in pot
Bring the 5 litres to boil, once boiling switch off heat
Add ~500gm LME, stir until completely dissolved
Return to the boil, stirring occasionally, being careful to avoid a boil-over
Once a 'rolling boil' is achieved, add 40gm Amarillo, boil for 60 minutes
With 10 minutes left of the 60, add 10gm of Amarillo
Once the 60 minutes is up, put the pot into an ice bath in my laundry sink, wait for it to drop to around 25 degrees.
Give the 7 litres of boiled water in my fermenter a good aeration
Add the rest of the LME (2.5kg) to the fermenter and mix it all in
Pour the cooled wort from my pot through a stainless steel colander into the fermenter
Check the temperature of the 12 litres in the fermenter. Add hot/cold water from the tap to get it up to 21 litres at around 20 degrees.
Pitch the re-hydrated US-05 yeast
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I'm curious on the remaining 2.5kg of LME, should I add that to the fermenter at the start, when i add the 7 litres of boiled water? Or should I add it at the same time as I add the boiled wort, at the end? I need the water hot enough for it to dissolve, so that's why i'm thinking to add it first....but then I have that mixture sitting in the fermenter for a couple of hours while steeping my grain then boiling the wort. Problem?