I think google is only 30/50m accurate as it uses free dataset. Anything less and there is a market that charges good money for it.
If you're that worried, then look at the surrounding area, fly to the highest point with your drone, note the highest object in the area you're going to be flying then set that as a minimum height + 10% for good measure.
You should be fine providing you stay below 120m, and don't take the piss with the autonomous modes.