"Expected return on Berkshire 7% real?
Tbills 1% real?

100% (+) Berkshire..."

I agree that given the high valuation of the S&P 500, BRKB/T-Bills is a better portfolio than S&P 500/T-Bills, although I don't think that BRKB will return 7% real, even though it has done so recently. Maybe 5% real over the long term. Over the next 5-10 years, though, I expect BRKB to return less than that, as the S&P corrects and BRKB falls with it. Over the next 5-10 years my guess would be something like 0%-5% nominal return for the S&P and slightly higher for BRKB. If we give the S&P 4% nominal, BRKB 5% nominal and T-Bills 4% nominal, that changes the recommended BRKB/T-Bill portfolio allocation to 25% BRKB/75% T-Bills and an S&P 500/T-Bill portfolio allocation to 0% S&P/100% T-Bills.

As we can see, the recommended allocations depend strongly on the expected returns of the S&P and BRKB. I hope that your expectations prove to be closer to actual than mine.

Good luck to all of us.