This ProBuilder code snippet demonstrates how to dynamically adjust the size of trading positions based on the Fixed Fraction method by Ryan Jones. The method increases or decreases the position size based on the strategy’s profit, allowing for more aggressive scaling in the early stages of profit accumulation and more conservative adjustments as profits grow.

once multiplier=1
once fraction=1000
once newlevel=1000
once oldlevel=1000
once startpositionsize=1
once positionsize=startpositionsize

if strategyprofit > newlevel then
    multiplier = multiplier + 1
    oldlevel = newlevel
    newlevel = strategyprofit + multiplier * fraction
    positionsize = multiplier * startpositionsize
elsif strategyprofit < oldlevel and multiplier >= 2 then
    newlevel = strategyprofit
    oldlevel = strategyprofit - multiplier * fraction
    multiplier = multiplier - 1
    positionsize = multiplier * startpositionsize
endif

Explanation of the code:

• Initialization: Variables such as multiplier, fraction, newlevel, oldlevel, startpositionsize, and positionsize are initialized using the once keyword, ensuring they are set only once and retain their values between different calls of the code.
• Increasing Position Size: If the strategyprofit exceeds the newlevel, the multiplier is incremented by 1. The oldlevel is updated to the current newlevel, and the newlevel is recalculated based on the current strategyprofit plus the product of the multiplier and fraction. The positionsize is then updated to be the product of the multiplier and startpositionsize.
• Decreasing Position Size: If the strategyprofit falls below the oldlevel and the multiplier is at least 2, the newlevel and oldlevel are recalculated based on the current strategyprofit. The multiplier is decremented by 1, and the positionsize is updated accordingly.

This method allows for a dynamic adjustment of position sizes based on performance, which can be particularly useful in trading strategies to manage risk and capitalize on profitable trends effectively.