If you are not tied to that model the SSasymp() model in R could be
considered and is easy to fit:
# to plot points in order
o <- order(cl$Area)
cl.o <- cl[o, ]
fm <- nls(Retention ~ SSasymp(Area, Asym, R0, lrc), cl.o)
summary(fm)
plot(Retention ~ Area, cl.o)
lines(fitted(fm) ~ Area, cl.o, col = "red")
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