First, you are using it with a dimmer switch, correct?
When using a dimmer, start low and slowly turn it up.
I would put over the middle/cool side. You want ambient temps up and cool side temps up.
By putting on the cool side, say 40% from the cool side (50% being the middle), you may get ambient temps up to 80-83 or so and the cool side to 77-80F or so.
If you need to move it closer to the cool side, that's fine, just make sure ambients are good and there is a gradient.
Foil is fine. Safer than a towel. Also, more sanitary.
Regarding height from screen, see what temps you get and if you have to move it closer, you can. Remember, every time you move the CHE and/or adjust the dimmer, you will have re-check temps and fine tune again.