Hi
If you are "Taking SP" (trigger action "back at SP") I do not think that can be done using a trigger condition.
What you are after can be done by placing a normal back bet at the known BSP as soon as the market turns in-play. You can then add any conditions you want, including setting a max price.
The attached trigger is an example.
It will place a normal back bet (not "back at SP") at the known SP price as soon as the market goes in-play. In the constant you can set the max price.
The possible problem with this method is it does not guarantee the bet will be matched whereas "taking SP" (trigger action "back at SP") guarantees the bet will be matched at SP.
If you try the trigger please use test mode.