The child orders of the bracket are submitted at the same time as the parent (i.e. opening) order. This means that at the time the orders are submitted is the fill price of the parent order not yet known. So you can't specify a price relative to the fill price of the parent order.
I think that there are two ways how you can circumvent this:
(1) submit a regular bracket order and specify a very wide stop price and take profit price. This to ensure that these orders are not hit immediately after the parent order gets filled. Wait until the parent order gets filled and calculate the correct values for the stop price and take profit price. Then modify the child orders to the desired price.
(2) submit the opening order and wait until it gets filled (thus not a bracket order). From the fill price calculate the desired stop price and take profit price. Now submit a pair of OCA orders, using these calculated prices.
