I would go for a 100yd zero.
Interestingly, I just took a look at four 500 No.2 rifles I have (Turner double, Field single and two Westley 1869/71 variants) and not one of them has a standing sight marked for any distance!
The Turner has a '2' on the first folding leaf which is not much taller than the standing sight so I'm guessing the fixed sight is for 100yds.