Because of the numb foot I detected while cranking it up at times in the previous spin, I took everything down a level today. The result shows in the final calories reading, which was about 420, where I think I may have hit 550 the other day? (Certainly over 500). For a reduced effort (objectively speaking) it was certainly hard going! I think maybe I'm holding back more than I realise, and this takes energy of its own to sustain.
It's weird that the "low impact exercise" gives me a numb foot, whereas the "high impact exercise" will clear away all hints of numbness for at least 3 hours afterwards. It must be a posture thing. I've taught myself to run with quite good posture, but obviously cycle like Quasimodo.
Anyway, box ticked, no numb foot, and less staggering, getting off the stationary bike. The moral of the story may be that it's wiser to slow down, no matter what you're doing.