toon, this is tricky stuff isn't it!! At least we have the benefit of being on the ground contemplating this, rather than rushing towards LAM at 7 miles a minute.
I am not an ATCO, but I know that this sort of question comes up quite regularly when they are discussing unusual circumstances.
For me personally the best answer is FL150, although FL80 comes a close second.
I am genuinely interested in other people's views, because I am sure that the solutions people adopt will be many and varied!
G W-H