Rear derailleur choice

[jaybeex2]

HI guys a little help needed, im rebuilding an 80's Raleigh Criterium 12 and have got to the gear side of things. Ive replaced the chain rings for a 52/36 Campgnolo set and the rear a Shimano 12/27. Ive noticed some ads give a maximum capacity of 27T how is this calculated, and would i need a long cage? Im looking to spend around £60 can anyone recomend anything? Im running friction levers at the moment.

Thanks in advance
John

 

[jaybeex2]

Due to the over whelming response to the post i bit the bullet and bought one, am now sorted thanks to all that took the time to reply.

JB