It is not that simple. Timeform ratings on the flat are approximately 6lb to 8lb higher for Group 1 horses than official ratings. This is from a post I made on the subject earlier this year.
Of the top 18 horses (rated @ 128 or above by TF in 2011)
Horse Timeform---OR---RPR
Frankel---143---136---139
Black Caviar---135---132---133
Dream Ahead---133---126---129
Excelebration---133---126---129
Canford Cliffs---133---127---130
Cirrus Des Aigles---133---128---130
Rewilding---132---127---130
Danedream---132---128---128
Strong Suit---131---123---126
So You Think---132---126---129
Deacon Blues---130---120---125
Workforce---130---125---128
Americain---129---123---127
Goldikova---129---124---126
Cape Blanco---128---122---124
Hoof It---128---118---124
Snow Fairy---128---122---125
Twice Over---128---125---126
Average 131.5---125.4---128.2
6.1 diff between TF and OR
3.3 diff between TF and RPR
