Mutt, possibly but tbh the instruction is usually 'ABC123 When ready descend FL180 Level By xxx speed on conversion (or in the descent) 280 knots or less'
Apologies that should have been in the original post.
I know using 'on conversion' gets on to the whole conversation of at what level does the conversion from Mach to IAS happen but when the aircraft has descended past the low FL200's I would really have expected the conversion to have happened.