Exponential is also overkill (even with jitter as others have mentioned).

I seem to remember there was a "you failed 5 PIN entries in a row, please wait 500000 seconds before you retry" on Apple phones. So, you probably also want a sensible max... which makes exponential a bit pointless. Just do a basic fixed delay + (large, e.g. 0.5 x the delay) jitter and you'll be fine for most things. You can add a bit of cumulative delay if it's really costly to do retries.