It took me ages to get through that page and my circumstances were similar. I tried so many times, I'm trying to think exactly how I got it to work!
First, I agree with #2 above, just put in an exemption claim to balance the gain, not the full 1270. I think the game changer for me was in the two sections below that again, where you put in the net chargeable gains for the period 01 Jan to 30 Nov, and again for period 01 Dec to 31 Dec (equivalent to lines 821 and 822 on the paper form). In both those sections for line (a) "Net gain to be charged at 33%", I entered zero in both and that seemed to satisfy it. Hope it works for you too.