Horse thinks "I see they want me to run 8 furlongs today. They’ve put 8 stone 7 pounds on my back. Well, I can do that. If I can get past that bruiser in stall 2, I might even try to get my nose in front, if the foreman wants me to. If they’d have put me in a 7 furlong race, though, I’d have happily carried
stones for them. They can forget next week’s race, though. They want me to run 10 furlongs with
on my back. What do they think I am, a carthorse?"
This would be a hard question to answer, because of all the factors ivolved in the race. Colts and geldings carry more than fillies, Old horses and higher rated horses would carry more than say 3y olds or lower class horses. Horses running over fences can generally carry more weight because they are older and can cope with greater distance.
I dont think you could say 8:7lb over 7f is the equiv of 8:10 over 8f because it depends on how fit, strong, age, sex, suited to the distance the horse is.
Does this make sense to you?