One alternative to estimations are projections via (for example) Monte Carlo simulations. I've been happily using https://getnave.com/ https://getnave.com/ for

One alternative to estimations are projections via (for example) Monte Carlo simulations. I've been happily using https://getnave.com/ https://getnave.com/ for that. The results seem to be in the same ball-park as my old estimations, but with less stress overall.