If all you have is the sub-assembly and the error is killing you, alter the fork length by a mm or two and the lower head angle will change accordingly atmo. PS when we were in London the lower joint was the first one produced. After it was brazed but still red hot, the head tube would be hung over a mandrel and the down tube was tugged up or down to conform to a series of angle templates that were sited against it. Different templates were chosen so that the desired head angle for the particular frame size would ensure that the first braze yielded the desired result. Yes - it was set while the sub-assembly was red hot, right from the firebricks it was brazed against. So very Dickensian.