you are correct that nose angle is the front/back angle of the disc whereas hyzer is the outter/inner edge.
as for how much is required, a lot of it depends on the disc.
general rule: the slower the disc and narrower the rim, the less nose down dependent it is. similarly, the less durable the plastic, the less nose angle sensitive it will be.
most putters/mids can be thrown nose neutral (flat) but perform slightly better w/ at least 1 degree of nose down. most older design drivers, e.g. pro d cyclones, dx gazelles will only need like 1-5 degrees of nose down. a disc like a champion orc will require more like 10.
most of the newer drivers: crush, starfire, orc, wraith, etc. will require a LOT Of nose down and are very nose angle sensitive, which is why i don't recommend them for players without a lot of power (generally lower-powered players do not have enough nose down).