When I try to add a breakpoint at 850 I get error "Cannot add breakpoint". It's much more than 25 px from other breakpoints (nearest is at 1050), and in Site Properties, minimum page width is set to 320. Why is this the case?
Not a solution, but just to make sure that you are on the right track
Also, can you post a screenshot for the benefit of other contributors that may want to probe further.