Hi. Yes thanks. Luckily I have a degree in Aerospace Eng - aircraft performance was something NATS didn't do well at. The whole point of a pilot transitioning between the speeds is so that they can fly under critical Mach number for the airframe or max q speed for the airframe depending on altitude (density etc which you mention). Whilst flying at the desired Mach number an eye can be kept on the ASI to stay above stall speed. The intersection of Critical Mach with stall IAS is the coffin corner you refer to. My point was that in the case of the 270 knot crossover at FL350 no special calcs are required. If the conditions mean the Mach number is safe when the ASI indicates 270 knots, then a 270 knot descent would be safe.