When a single timer tick last less than the adjustment step time
(i.e. < 500 ns), we end up with a zero adjustment value which either
leads to a spurious early shot detection due to a rounding error, or a
null gravity value at the end of a lengthy but useless calibration
process.

Make sure to adjust by at least one timer tick, and mitigate rounding
errors when checking for early shots when the timer frequency is lower
than 1e9 / ADJUSTMENT_STEP.