Don't confuse the default flight path your character follows with a given amount of knockback for the attack's actual default trajectory. The vast majority of attacks have a 45° angle, and so you would generally survival DI those full diagonally up & towards.
To more accurately see the attack's actual trajectory use it on Jigglypuff (floatiest character), and with extremely powerful knockbacks WAY beyond what would be needed to kill you (either by being at crazy high damage or for fixed knockback attacks you can increase the handicap and offense ratio).
Full strength DI changes the launch angle by about 17.5° in either direction. This is only where it's possible to get a full strength input perpendicularly.
For angles that don't fall on one of the 8 cardinal/ordinal directions (notches) of the control stick you can't actually get full strength DI due to the controller's design. By applying a more perpendicular angle, you sacrifice distance of input/strength of the DI since you can't actually press the stick in that direction as far as you can in one of the 8 directions. Attacks with these intermediate angles generally end up having the 'ideal' angle for DI be more towards one of the 8 directions rather than being the true perpendicular.
Angle ... Strongest DI ........ Examples
-----------------------------------------------------------------------------------------------------------------------------
+90° .. [DI: ±180°/±0°] ...Peach U-Smash(Shoulder); CF/Ganon/Samus/etc D-Air(Ground)
+85° .. [DI: ±180°/±0°] .. Fox U-Air(Kick2); Marth U-Tilt(Above,Behind)
+83.75° [DI: ±180°/±0°] .. Falco Dwn-B
+80° .. [DI: ±180°/±0°] .. Fox U-Smash(Strong); Ganon D-Tilt(Outside), F-Smash(Up); Peach U-Air
+76° .. [DI: ±180°/±0°] .. Kirby D-Smash(Inside)
+75° .. [DI: +150°/-30°] . Falco D-Tilt; Marth D-Smash(Strong-Back); Peach U-Smash(Weak)
+70° .. [DI: +150°/-30°] . Ganon D-Tilt(Inside), F-Smash(Normal); Falco U-Air(Kick2-Strong)
+65° .. [DI: +150°/-30°] . Link D-Air; CF(Nipple)/Falco/Marth(NTSC) D-Air(Ground)
+60° .. [DI: +140°/-40°] . Doc F-Air; Ganon F-Smash(Down); Bowser F-Smash(Outside)
+55° .. [DI: +140°/-40°] . Ganon N-Air(Kick1)
+45° .. [DI: +135°/-45°] . {WAY too many to mention}
+40° .. [DI: +135°/-45°] . Sheik D-Smash
+32° .. [DI: +130°/-50°] . CF F-Air
+30° .. [DI: +130°/-50°] . Marth D-Tilt; YL/Yoshi/Zelda D-Smash; Ganon/CF U-Air(Behind-Above)
+29° .. [DI: +130°/-50°] . Peach F-Smash(Racket)
+27.5°. [DI: +130°/-50°] . Doc B-Air(Strong); Kirby D-Smash(Outside)
+25° .. [DI: +120°/-60°] . Sheik F-Air; Fox/Falco D-Smash
+24.5°. [DI: +120°/-60°] . Bowser B-Air(Strong)
+20°... [DI: +120°/-60°] . Kirby D-Tilt
±0° ... [DI: +90°/-90°] .. Fox Dwn-B; Ganon(Behind-Below)/Pikachu(Overhead) U-Air; Jiggs D-Smash; Link Up-B(Weak)
(±180°=Towards Opponent, +135°=Up&Towards, +90°=Up, +45°=Up&Away, ±0°=Away, -45°=Down&Away, -90°=Down, -135°=Down&Towards)
Here's a more filled out list of angles (though not nearly close to being complete):
Code:
Bowser F-Smash (Inside) [+45°]
Bowser F-Smash (Outside) [+60°]
Bowser B-Air (Strong) [+24.5°]
Captain Falcon F-Air [+32°]
Captain Falcon U-Air (Weak) [+30°]
Captain Falcon D-Air (Meteor) [-90°; +90° grounded]
Captain Falcon D-Air (Spike) [-65 deg; +65° grounded]
Captain Falcon D-Throw [+65°]
Captain Falcon U-Throw [+85°]
Doc F-Air (Outside) [+60°]
Doc F-Air (Inside) [+60°]
Doc B-Air (Strong) [+27.5°]
Doc B-Air (Weak) [+45°]
Falco Dwn-B [+83.75°]
Falco U-Tilt (Inside) [+97°]
Falco U-Tilt (Outide) [+90°]
Falco D-Tilt [+75 deg]
Falco F-Smash (Front) [+45°]
Falco F-Smash (Inside) [+110°]
Falco U-Smash (Strong) [+95°]
Falco U-Smash (Weak) [+45°]
Falco D-Smash [+25°]
Falco U-Air (Kick2-Strong) [+70°]
Falco D-Air [-65°; +65° grounded]
Fox U-Smash (Strong) [+80°]
Fox U-Smash (Weak) [+45°]
Fox D-Smash [+25°]
Fox U-Air (Kick2) [+85°]
Ganon D-Tilt (Outside) [+80°]
Ganon D-Tilt (Inside) [+70°]
Ganon F-Smash (Up) [+80°]
Ganon F-Smash (Normal) [+70°]
Ganon F-Smash (Down) [+60°]
Ganon D-Smash (Kick1) [+160°]
Ganon D-Smash (Kick2) [+120°]
Ganon N-Air (Kick1) [+55°]
Ganon N-Air (Kick2) [+45°]
Ganon U-Air (Semi-Weak) [+30°]
Ganon U-Air (Weak) [±0°]
Ganon Fwd-B [+105°]
Ganon D-Throw [+100°]
Ganon U-Throw [+90°]
Kirby D-Tilt [+20°]
Kirby D-Smash (Strong-Outside) [+28.5°]
Kirby D-Smash (Weak-Outside) [+28.5°]
Kirby D-Smash (Strong-Inside) [+76°]
Kirby D-Smash (Weak-Inside) [+76°]
Link D-Air (Strong) [+65°]
Link D-Air (Weak) [+65°]
Marth U-Tilt (Strong-Front) [+110°]
Marth U-Tilt (Strong-Above) [+85°]
Marth U-Tilt (Strong-Behind) [+85°]
Marth U-Tilt (Weak-Front-Inside) [+45°]
Marth U-Tilt (Weak-Front-Outside) [+110°]
Marth U-Tilt (Weak-Behind) [+85°]
Marth D-Tilt (Both) [+30°]
Marth F-Smash (Strong) [+45°]
Marth D-Smash (Strong-Front) [+70°]
Marth D-Smash (Strong-Back) [+75°]
Marth D-Air (Strong; NTSC) [-65°/+65° grounded]
Peach U-Air (Leg) [+80°]
Peach U-Air (Crotch) [+45°]
Peach F-Smash (Pan) [+80°]
Peach F-Smash (Racket) [+29°]
Peach U-Smash (Shoulder) [+90°]
Peach U-Smash (Weak) [+75°]
Peach D-Smash [+140°]
Sheik D-Smash (1st Hit) [+40°]
Sheik F-Air [+25°]
Yoshi D-Tilt [+28.5°]
Yoshi D-Smash (Both) [+30°]
Young Link D-Smash (Both) [+30°]
Zelda D-Smash (Front) [+30°]