Hold down locations used to be closer to 3rd points but in recent years fabricators have seen typical hold down locations at 0.4 and 0.6. We prefer 0.4 and 0.6 since it flattens the angle of the strand out more, reduces the uplift force at the hold down, and may allow us to place the strand hold down at one location vs. breaking it up into more locations which requires having to add hardware.
While a ratio of the length can certainly be used in design, it is recommended that a fixed value be used for a set of similar girders so that the hold down location is not different for each girder or group of girders. This simplifies detailing of plans as well as fabrication. Fabricators may submit changes in hold down locations in their shop drawings to fit their available hold down locations. When reviewing shop drawings from fabricators, designers should allow ample tolerance for shifting a hold down location to avoid supports of the formwork or other spacing considerations for the hold down mechanism in the bed layout of the fabricator.