For me I base it on how closely it resembles previous form, so if they have proven to be a fairly reliable guide in the past. Lord Snootie had RPRs of 122, 125 and 121 at the end of last season then a 114 to start this year, but would probably have been another low 120 but for making a bad mistake. He has gone up in weight slightly but beaten 8 lengths in a grade 3 handicap, I don't think it is unreasonable to think he has once again ran to something in the mid to low 120s. Santos Blue would be rated partly through Crambo as they have correlative form. Ignoring the jockey claim, SB was getting 4lb from Crambo yet was beat 2 1/4 lengths. with a further 6lb swing he is now beat 6 lengths. Either Crambo has improved enormously on his run, Santos Blue has got decidedly worse or it's a combination of the two. That is where we can use other horses to help guide.
I tend to ignore those beaten a long way as they are less likely to have run their race. Chances are they have been eased down or something has gone wrong for them to be beaten so far. 8 lengths in a three mile race isn't that far really. I would consider all those down to 9th here, beaten 11 3/4 for having run their race just with excuses as to why they haven't performed to their best.
