12m is going to pretty short for flying a STD. The shorter the lineset the smaller the window and the less margin for error. If you're flying a STD you'll need some wind ... which will further reduce the margin for error. People who fly on short lines generally use ultra lights or super-ultra-lights / indoor kites - unless they have other-worldly skills and reflexes (Chris Goff).
Others may disagree but I'd suggest at least 50ft and preferably 60ft for a STD. So make a 2nd, slightly longer set up too