When computing the suggested switch depth for a gas,
we should take the next stop depth above the MOD, i.e.
round down. Otherwise we can produce MOD violation warnings.
We need, however, a bit of fudge as otherwise we do not
suggest to switch to o2 at 6m.
TestPlan uses the MOD to determine the depth to switch to
Tx21/35. This happens to be 65.378m. Therefore, switching
at 66m violates the MOD, the switch should be at 63m.
This then affects the gas usage.
Signed-off-by: Robert C. Helling <helling@atdotde.de>