[V8] cooling system thermostat

[Ed Kellock]

How far should the thermostat open when it reaches its operating temp?
 
I have an 87C thermostat in 90C water and it doesn't look like it's open
that far, but I'm not sure how far it should open.
 
Thermostat is a new one from that I just removed from a 90 V8 that I just
completed the timing belt water pump etc. job on.  
 
I flushed it very thoroughly with using 8 gallons of distilled water then
refilled with the appropriate amount of Pentosin antifreeze and distilled
water.
 
Yesterday it was in the low 90's and the temp gauge was at 100 or just above
at steady state 45 mph and would climb to 105 at a signal or climbing even a
moderate hill.  This car never ran that high before and neither does my
other V8.