TBH I've lost track of whether the OP was looking at the planning case or what to do in the real world post despatch ( as you imply either the FMC will have an eng out option or failing that once stabilised post drift down there should still be enough info available from the FMC and the Fuel Flow gauges (s) to calculate fuel overhead the alternate
).
Back in the initial post the first query seemed to actually be an ask about the 60 minute rule and the ERA, but then we got into Crossfeeds...and here we are...
