make sbi_laterN compute and rely on that instead of MakeLaterN

With a pretty proof by Robbert
5 jobs for ci/ralf/telescopes in 32 minutes and 24 seconds (queued for 2 minutes and 39 seconds)
Status Job ID Name Coverage
  Build
passed #15711
fp
build-coq.8.7.1

00:05:09

passed #15710
fp
build-coq.8.7.2

00:05:30

passed #15709
fp-timing
build-coq.8.8.0

00:05:11

passed #15708
fp
build-coq.8.8.dev

00:06:18

passed #15707
fp
build-coq.dev

00:30:20