I like to see three numbers.
1 - Output shaft angle
2 - Driveshaft angle (installed, weight on wheels)
3 - Pinion angle
Under hard acceleration output and pinion angle should be within 1 degree, but it's often difficult to determine that, so some fudge is added based on what you think you have for pinion movement. The trade off for u-joint plane misalignment is vibration
Additionally, the driveshaft angle, when compared to the other angles, determines u-joint operating angle, and Spicer recommends max 3 degrees as a conservative number for any speed a car may likely go, there but never under .5 to ensure the u-joint rollers keep moving. Sometimes that is not achievable, so you need to look at expected driveshaft speed. The trade off for more operating angle is shorter u-joint life and vibration
With 5th gear and a decent gear ratio, your engine speed may be well under control, but the driveshaft speed will be higher, so the more care you put into this, the better. An example of a 3.70 geared car with 26 inch tire, is a driveshaft speed of 4783 at 100 mph. (shorter tires and more rear gears will be worse)
Just because I have some Spicer tables that address this, I'll use Spicer again, they recommend a max u-joint operating angle of 3.7 degrees for a 4500 rpm driveshaft speed and 3.2 degrees for 5000 rpm.. So there is some fudge factor from the original 3 degree recommendation, but remember, that RPM ignores the 5 speed, and those of us with overdrive can easily hit that driveshaft speed. (4780 * .82) = 3919 engine RPM with a .82 OD, and 3059 engine RPM with a TKO-600 with a .64. Even if you aren't driving around your local area at 100 mph, keep in mind, engine RPM in 4th as you enter the traps = driveshaft speed, so 4700 is realistic
So without knowing your driveshaft angle, we don't have all the pieces. However, I can say, compared to the body, the stock output shaft angle on a Mustang is 3 degrees, and if you have a TKO crossmember that didn't require trimming of the floor, your numbers make sense. I chose not to run it that way and added .750 (I think, it's been 12 years) under the transmission mount to get there.
What I have been doing since the Mustang install is start with a measurement from the center of the output shaft to the floor before I pull the old factory tranny. Then I match it, and even with some slight differences in length depending on spacers, bellhousings etc, I am usually very close and usually exact where I want to be.