Need to add ordersize option to my EA that calculates lot sizes according to account size

In My EA now I can input lot sizes manually.  I need to keep this option but add another way of calculating order size.  This EA needs to calculate lot size according to the risk set by the stop loss.  I would like to be able to specify in the input section how much risk to the account in % I am willing to take.  For example if my account size is $1000 and my stop loss is set at 100 pips,  this ea should be able to calculate order size only risking the percentage of the account specified in the input section.