I have written grants for years. I believe I have been fairly successful. My grant submission in relation to funding rate averages from 6-12%. However, I just applied for a full-time position where the organization listed that it REQUIRED applicants to have a history of a 60% funding conversion rate for grants submitted.
And my reply to the reader (in part) was:
The industry rule-of-thumb I've heard is that 1 in 12 proposals gets funded, or about 8.5%, and that's in a good year.
A professional grant writer should be able to do better than that, hopefully even one in four or better, but it really all comes down to the organization they're writing the grant for, and that organization's reputation and existing relationships with foundations.
An established, larger, or older nonprofit might be only working with the same foundations year after year, and only responding to direct invitations to submit a proposal. In such a situation where nothing is sent out as a "cold call," a 60% success rate might be easily achieved, or even exceeded.
Meanwhile, a newer, start-up nonprofit might be very happy with results of one in 15 proposals being funded, as virtually every proposal or LOI they send out is an introduction to the agency and an attempt to just get a foot in the door. Relationships with foundations have to start somewhere, and the LOI is traditionally that place.
So, your question was, is it reasonable for a prospective employer to "require" a 60% conversion rate. My answer is simply to turn it back onto them. What is their current conversion rate? Do they have established relationships with funders or have they had scattered luck?
If you're preparing for a job interview, you can figure out some of those answers by going to guidestar.org and downloading their 990 tax returns for the last few years. Check out who is funding them, and whether the list is completely different each year, or from a stable group of sources. Are they large grants, small grants, what percentage of the budget is funded through grants?
Meanwhile, in your letter and resume, you should explain that your success rate is based on the assignment given. If they client asks for LOIs to be sent to "a dozen new funders" that it will naturally be less successful than when a client asks you to write for a specific funder who has requested the proposal.
If they don't like that explanation, then, frankly, you're better off not working for them. If your job performance is going to be judged by an unrealistic goal your tenure will be short, stressful, and unhappy. Accepting a job you can only fail at is never a good career move.