One thing I would like to change is the Margin for the asset "USOil" only.

I would like to reduce it by a factor of about the current oil price in USD, which is about 45.

I have tried the following which seems to reduce the margin for all assets by a factor of 45. This seems to have the effect of reducing all position sizes by a factor of 45, not just the oil asset.

Capital = 50000;
if(asset("USOil")){
Margin = 0.01 * (Capital + (WinTotal-LossTotal))/45; }
else {
Margin = 0.01 * (Capital + (WinTotal-LossTotal));}