To the op, in my opinion, the ''short letter'' rule is the greatest fallacy in ttm autograph collecting. Now, the reality is most celebs probably do not even read fanmail. So you're probably wondering why even bother with writing a good letter? Well a while ago, a band tour manager posted on here. He said that ALL fanmail sent to the concert venue goes through him. If he feels that the person requesting the auto is an ''ebayer'',he bins it. No ifs or buts. Now,it is reasonable to presume that such a policy applies via database addys. Whilst they may only send out pre-signed pics, I think they still scan the fanmail to remove the ''ebayers''. For example, nowing that two prominant singers frequently send out autos pre-signed, I decided to simply write a paragraph for each request. Guess what? Never heard back from either of them.
Also,if you decide to send vv, it seems to be the go for a more ''personal'' response from a movie star for various reasons. I cannot fathom why you would waste the chance of getting a once in a lifetime response for the sake of writing a ''short letter''? My two best andd most cherished responses ever came from, you guessed it, two page-letters sent vv. The first was Gemma Arterton. I had only seen her in ONE movie at that point, but I absolutely loved her in it, and wrote a passionate two page letter. As a result, she paid for postage, supplied a signed picture AND wrote me back a note thanking me,and answering my five questions. The second was Slumdog Millionaire star Freida Pinto. She had only ever been in the aforementioned movie at this point,but I was still able to write a two page letter. I wrote about myself, about why I love her as a person (beauty,talent, a sweet personality), slumdog millionaire and requested my items signed. Each of those topics was roughly half a page. She replied by signing and personalissing all my items with beautiful dedications (even ones that I did'nt intend on being signed!), wishing me a very happy birthday and wrote me back a short,but very very kind and sweet note on the back of one of my pics. Oh,and she supplied the return envelope,paid for postage and included cardboard so it would not bend! Why did these two beautiful stars go out of their way to give me such a wonderful reply? They are obviously extreamly kind people, but it's because I went out of my way to write a heartfelt letter!
Good luck

Best successes: Gal Gadot, Jennifer Lawrence, Catherine Zeta-Jones, Rachel McAdams
Best Responses: Freida Pinto, Gemma Arterton, Berenice Marlohe, Jeremy Renner
Recent successes: Connie Nielsen, Ursula Corbero, Daniela Melchior, Kate Beckinsale, Amber Heard