If you've ever paid attention to how fast the Fastpass lines move compared to the standby, that is because they're filling 80% of the seats from the Fastpass line. If there was only one line, it would move even faster than that.
That's not true. Disney can dedicate any percentage from 0 to 100% of an attraction's capacity to FP, however the amount will vary greatly based on the attraction, overall capacity, average up time, staffing, and maintenance.
Disney does not release all of the planned FP reservations immediately. They're released in blocks for a good reason. A certain percent are made available 60-days out, with the rest released between then and up to an hour before the time slot. Disney has to be able to account for how the attraction will operate. Not all ride vehicles may be availble. Also, the ride may not be fully staffed for all operating hours affecting capacity.
Let's take Dinosaur at Animal Kingdom for example. Early in the day they may not open both loading-unloading platforms and run fewer vehicles. The same may be true near the end of the day.
Also Disney will vary the amount of FP reservations made available by hour. Typically you may give a higher percentage of a ride's capacity to FP early in the day when the park is less crowded and less later in the day when it is more crowded. You also want to reduce the number of the reservations later in the day to account for the unexpected.
Why is that? Let's say you allowed FP reservations for 80% of an attraction's capacity for every hour of the operating day. What happens then when the attraction experiences downtime? What happens when some of the rides vehicles have to be unexpectedly pulled from operation for maintenance reasons? Also, how do you accomodate the downtime of other attractions in the park? Yes, the downtime of other attractions impacts FP at other rides. If you hold a reservation for an attraction that is down then you get a replacement FP that can be used at any time thereafter for an alternate attraction or for the same attraction you reserved for. Disney also has to account for customer service FPs?
Another reason why you don't dedicate 80% is because of the hour window. There is an unlikely, but possible scenario where enough FPs could show up all within a few minutes and cripple the attraction. It would stop the stand-by line and make the FP line equally long.
Under most scenarios it's fair to say that about 50% of a ride's capacity may be used for FP reservations, not 80%. However the actual number is less than 50% on many attractions. Let me also clarify...there is a difference between % of capacity made available for reservations versus % of capacity actually used for FP.