Do you have a longer mirror somewhere else in your house? I end up diffusing my hair in front of a floor-length mirror I have outside my bathroom. That way I can see myself when I'm diffusing upside down.
Our porosity/drying times are different, but I usually leave the sections in the diffuser for about 45 seconds at a time. I start out on my dryer's warm setting, and after my hair starts to get dry and I can feel the warmth all the way up to my scalp, I start holding the cool blast or switching to the no heat setting. I'm not super precise about the timing aspect, I've kind of learned to do it by feel now.
Also, I find finishing by holding the dryer a few inches above my head, pointing down (as in, no hair in the diffuser, just blowing air down at my head) using the cool setting calms down a lot of the little frizzies.
2b/c, med to high density
LCLF: fine, medium porosity, normal elasticity
Mod-CG: low poo & water washes, no co-washes
no sulfates, no silicones, no polyquats
HGs: FSG, gelatin PTs, pixiecurl diffusing
Growing out a chin-length bob, just reached collar bone length!