The hullabaloo about statistics has been very interesting.
The idea you'd want to know my closing ratio (meaningless as I've said) or what percentage of projects I sell is pretty funny. It implies the best indicator for whether I can sell your book is either how many books or what percentage of my list I've sold. That's utter dreck. Some of the most effective agents working today sell less than ten books a year. Some of the least effective have sold a lot this year cause they are second/third books on multiple book deals and they didn't have to do a damn thing to ink the deal (generally speaking of course).
And, to be a number with any meaning, you'd have to know what other agents are selling and how much they represent, and what's "normal". 40% is a crappy test score but it sounds a lot better expressed as a .400 batting average. One agent's numbers are meaningless unless you have something to compare it to. (There are no benchmarks in publishing because agents don't collect, let alone give out, that kind of info).
And just to put the final kibosh on this, you have NO way to verify any of that. I could tell you I sell 100% of the works on my list. How would you know any different? AuthorHouse publishes 99% of the work sent to them. Are they "better" than FSG which publishes probably .01% of the work sent to them?
Here's a question an author should ask: how many publishers or editors are there who buy this kind of book? How many of them do you know? How many of those editors have bought things from you?
If an agent is consistently selling your kind of book and she wants to represent you, you'd be an utter fool to say no cause she's only sold 20% of the books on her list.