The way to properly calculate the safe working height of any given setup is to know the specifics. Since we know some of the specifics already we can make a few rough guesses. If you are only hanging (2) S4 lekos I will make a dangerous assumption that your Tbar is around 2' long. Considering the fact that I have no idea if you are trying to avoid lighting anything inbetween the lights and the backdrop (i.e. people, tables) I woould say that the best angle for the lights are 40 to 45 degrees so if you use a 15' upright pipe you should be ok with a couple of sandbags.
The dislaimer here is for all of the unknown variables: If people will be walking around your setup, you need it be alot higher, you are lighting it from the side, etc. Be safe, use good judgement, never ask strangers for advice when a potential outcome is death,