Here's what I did:

I bought 8x10 window glass at the hardware store and washed with Dawn liquid soap and hot water. Scrubbed, rinsed, dried on rack.

I took Rockland Colloid Ag+ emulsion (from Freestyle) and heated the bottle upside down under hot water. (Approximately 140 deg. F)

In a small glass graduate, I mixed 5-10mL of Photo-Flo with warmed emulsion to a total volumn of about 45-50mL. This was then poured on the dry glass plate starting in the center and spread in the style of wetplate collodion.

Here's 2 results: one of the worse and my best.

Unfortunately, my 8x10 plate holders don't fit my 8x10 camera, so I haven't tried the negs. But at least I have a rough idea of emulsion volumn for 8x10, and I found I don't absolutely need a subbing layer.