If you had already published a book by subscription, & could tell me the number of copies sold, I would then know what to say. (I say by subscription because books published in “the trade” would not be of use as evidence.)

In the absence of such data, I can only advise you to take a royalty—not less than 5 per cent of retail price of the book—& of course as much more as the publisher will pay.

When books are sold outright to publishers, somebody always gets hurt. It is not usually the publisher.