Well 5.7 should be ok depending on your plants genes. But I think its safe to assume that you got too much nutes in there, water evaporates and is used by the plant, so ur EC will always rise over time. Really you should invest in one, I have seen decent ones for $10 online. Best solution I would say, is to flush the plant, and replace the nutrients with a 0.5 mix. And redo that every week until u get an EC meter. You will get much better results with slightly too little nutes than with too much.
edit - if you don't trust ur pH meter, keep pH at 6 until u can get hold of some calibration solution.