Spec Threshold calculation for babe

[drskalman]

it should be consistent across implementation. The rust implementation is here
https://github.com/paritytech/substrate/blob/master/core/consensus/babe/src/lib.rs#L1022

[FlorianFranzen]

The threshold computation is specified in 6.2.2., so I think we should be able to close it.