Rank Structure of the Late Roman Legions

[D B Campbell]

So it would appear that the ratio of cavalry to infantry seems to have been 1:2.

Careful, Scott. The ratio of cavalry unit-size to infantry unit-size might be 1:2, but there were still more infantry units than cavalry units.
For example, at Strasbourg in AD 357, Julian fielded around 10,000 infantry and only 3,000 cavalry.

... praepositus, a term I haven't been able to translate or anglisise let.

Praepositus simply means "one who has been placed in charge". It wasn't actually a rank -- legionary centurions were often "placed in charge" of auxiliary units -- but more of a function. Hence, a man with the rank of tribune could be described as the praepositus of his unit.

As with the comitatenses it is difficult to estimate unit sizes. in the third century cohortes and alae had a nominal establishment of 480, though John Lydus in the sixth century records alae as 600. The few military units recorded in the Notitia may have been around 750 strong, as in the third century. Border equites and auxilia were probably the some size as cohortes and alae ...

You've got a crucial typo in your quote. Elton is referring to milliary units, which were nominally 1000-strong. He makes the point that their normal complement may have fallen to 750.