is it that difficult?
When you dial in FL 300 on the MCP,as you would HAVE to do at Top of Descent short of XXX, the new MCP altitude (30000) automatically appears on the scratch pad in Cruise page, just begging you to enter it into the cruise altitude line. This done, and the altitude at XXX set at FL 300 as you describe, would get you an idle thrust descent to XXX/ FL 300 and subsequently another idle descent to YYY/ 280.
Or do I have it wrong somehow.. A new NG driver myself.