Can anyone help me?

I am trying to change the prompt_text of items using SET_ITEM_PROPERTY.
The prompt text changes ok but the alignment goes screwy!
It needs to left aligned, I have tried setting this at the same time and it does not work.
I think that the prompt_edge_offset property needs to be set but how can I find out what the value needs to be.
eg.
the original prompt_edge_offset has a value of 75, but when I change the prompt text the 75 value puts the prompt in the wrong place.

Over to you
Cheers