Hello Mark,
Good impedance calculators are somewhat uncommon. Several free calculators are available on the net, but most good calculators charge for their use. A couple that come to mind are Polar Instruments, and UltraCAD. I believe Polar has started a "pay as you go" service for those with lighter use requirements. It has been my experience that both of these offer pretty accurate results. There are of course others available, some are easier to use than others.
Circuit length can be ignored as it is not a factor in solving for characteristic impedance, but length can make the need for impedance control less important in certain circumstances. In a low frequency application with very short trace runs (< 1/2") the impact of signal discontinuities caused by vias or component terminations becomes more of a problem than the circuit boards construction. I'm not saying you can ignore it all together, just that as trace runs become very short characteristic impedance of the flex has much less impact in signal integrity.
Just an added thought on impedance testing of actual boards, flex or rigid. Characteristic impedance is indeed just that, "characteristic". Often it is difficult to test the actual circuit pair due to conductor layout or inaccessible features. Impedance testing should be done on a coupon built on the same process panel as the circuit. One test per panel is sufficient. Testing every differential pair may be possible but economically unviable.