My copy of Covington’s book suggests that at f/11 and 400ASA/ISO, an exposure of about 1/250th is about right for prominences and 1/15th for corona. Well worth bracketing from 1/1000th down, but how are you mounting the camera? Driven? Undriven. Fixed tripod?
Handheld and without autofocus and stabilisation, the suggested minimum exposure for a half decent photographer in the film days was 2/FL where FL is focal length in mms or, in this case, 1/200th of a second. But a tripod and cable release should improve on that by nearly a factor of 10 I think.
For 1999 I went from 1/1000th to 1/20that 400ASA. Pics were a little soft, but usable.
But really we need an expert like Nick James who has seen total eclipses more times than most. 🙂