Real-world experiences: '64 Corvair, light in the nose @ 65MPH; added 1-st-gen Z28 ft spoiler, problem solved. '65 Belvedere A/FX car, 157MPH max at the stripe no matter what; added 3" spoiler to trunk, 45* angle, went almost 170. Turned out it was a lift issue, causing tirespin. '73 911, similar issue as Corvair, added 911Turbo ft spoiler, worked as on Corvair, but rearend got looser. Needed Carrera ducktail spoiler or Turbo spoiler to balance; fairly common experience among 1st-gen 911 owners.
Note the early TransAm cars; those spoilers were considered mandatory by the teams for aerodynamic benefit.
The idea is to minimize air under the nose (lift), air getting under the car (drag), and to get the air onto the trunklid (lift) and reduce turbulence behind the car (drag). The effects are generally measured and noticeable at highway speeds in the dozen or so articles and tests I've read. Should be easy to google the info.