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how do you do set it?

Well, exactly how the magnification is handled is also important. On some programs, a magnification of a gives you a picture a times smaller; on some, a magnification if a gives you a picture 2^a times bigger. I think that, assuming the magnification works proportionally, you want a max-iteration proportional to a; if that doesn't work, the maybe 2^a. But it's very important to change this depending on where you are. Zooming in near (0,1), (-0.75,0), and (0.25,0) will all require totally different growth factors.