from the WIKI:
DOUBLE_UP_CAP 1 Values: numerical - represents a ratio. This defines the ratio to the pairs balance to be used for each consecutive buy when doubling up. Setting it to 0.5 would mean it uses a 0.5:1 ratio for averaging down. It is recommended to set this as high as you can afford, to increase your chance to actually average down and sell at profit. Example with ratio of 1: initial buy of 100 LTC, first double up buy order is 100 LTC, second will be 200 LTC, then 400 LTC, etc. Example with 0.5 ratio: initial buy of 100 LTC, first double up buy order is 50 LTC, then 75 LTC, then 112.5 LTC.
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I personally do not play with double up. I wait my bags out. Double up can be a very quickly acting tool. As I check my bot only once in two days (I have it running on a VPS) or even less, double up is too quick for me. I disabled it on all my strats.