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If someone asked me to proofread in return for a percentage, I'd turn them down, because I'd want to be paid as soon as the work is done, and I wouldn't want how much I get paid to be dependent on ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/26887 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/26887 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
If someone asked me to proofread in return for a percentage, I'd turn them down, because I'd want to be paid as soon as the work is done, and I wouldn't want how much I get paid to be dependent on sales of copies. However, if you want to take them up on it, I guess you should first work out the flat fee you'd be willing to work for, then work backwards from there. Where _n_ is a reasonable estimate for number of copies the book is likely to seel, _p_ is the net profit on each copy of the book, and _s_ is the minimum sum you want to be paid, then the percentage you should ask for is _s / p \* n \* 100_. The tricky bit is _n_, as you can't know for sure, but you could look at sales figures for similar titles in that subject area and correlate it with sales of the author's previous titles, maybe?