If you think about Zentangle as a meditative art form, how can you not think of mandalas?  Zendalas can be drawn on any paper, and Zendala tiles are also available.  You can create Zendalas by carefully measuring to find the centre of the circle and working outwards, and some people like to use a compass to divide the sections.  While this might not look like ‘real’ Zentangle practise, it depends on how you feel about it.  Some people also create their Zendalas without a ruler, just letting the creative flow lead them, and ignoring the fact their zendala won’t be exactly symmetrical.  I’ve also seen beautiful tangling work done on a Zendala tile that just starts at one end of the circle and grows organically.  If we truly mean there are no mistakes, then the rules themselves must also be flexible to fit our own explorations.