Anyone found that perfect multiplier number to stop farming?

Yes, I have all 5 fully upgraded. That brought me across the 300% barrier (excluding SH%). I'v built and placed all SH houses etc., and I realise that the earning goes faster, but as said, it's about 20 Donuts a day, not more, harvesting always Kwik-E-Marts every 4 hours. I always build 21, which now amounts to roughly 1.25 Mio XP plus 3-6 donuts, depending on whether there is one rollover during harvesting or two.