I would use delta P and a barometer that I have on me.
I have two barometers with me at all times, my watch, my phone, I'm pretty sure my ipad has a pressure sensor too.
This requires a bit of knowledge about standard atmosphere, but hey we're professionals.
FL30, 60, 100, 140, 180, 240, 300, 390 (Easy to memorize, +3, +3 then +4 +4 +4, +6 +6 and +9)