Paint and build a front racked town bike

Ok, I've done some maths with some numbers I found on the Internet.

Premises:

• 8 speed cassettes space the cogs at 4.8mm to centre
  

• 9 speed cassettes space the cogs at 4.34mm

• standard shimano pull ratio is 1.7. That is, the derailleur moves 1.7mm for each 1mm the shifter pulls the cable

• old Dura Ace pull ratio is 1.9.

Therefore:

• a 9 speed shifter pulls 2.553mm of cable per click
  
• an 8 speed Dura Ace Shifter pulls 2.526mm per click
  
  

Its close, less than three hundredths of a mm per click discrepancy, but it adds up to 0.21mm of cable over 8 clicks, and with 1.9 ratio, that's closing in on half a mm out of place at the derailleur.

While clearly not ideal, that still doesn't sound like a lot. And you'd spread it so it was a quarter mm out at each end of the range...

Anyone done this? Unfortunately I don't have an 8 speed wheel (though I have some 9 speed shifters), so I'd have to invest in more than the derailleur to test it out. If it worked well, I'd expect there to be more advice available suggesting doing it.