We don’t have a direct Risk Factor that calculates the lot size by percentage; however, the closest method will be "Auto Risk". It depends on the risk you take on the master trade. If you take a risk of 1% on the master side, you have to use the Auto Risk with a value of 1, if you risk 2% on the master and want to risk 1% on the slave, you have to set the Auto Risk with a value of 0.5.
With Auto Risk value, 1 will take the same risk, 2 twice the risk, 0.5 half the risk,...