please elucidate how you suggest to measure the %error
why bother, there will be none. The only precision component needed is Rfix (see below). I would make Rfix out of two resistors in series with opposite tempcos, that way you can get virtually 0%.

make a setup like this (example illustrates reading from a voltage divider, you, probably use a bridge, but same method applies)

Vsomething
  *--------------------- ADC1
  |
  Rfix
  |
  *--------------------- ADC0
  |
  Rvar
  |
gnd

with this setup adc1 will read x and adc0 wull read y

so whatever Vsomething is (cheap reference) 

x     (Rfix + Rvar)
--- = -------------
y     Rvar 

Thus, the only thing affection your precision will be Rfix which you will need anuhow, the error from Vref has disappeared.

Erik