Just received my first 500kHz report from outside England at -26dB S/N from OR7T in JO20ix which is 337kms away.
Using the formula in the RSGB LF Handbook I just calculated my ERP on 500kHz assuming an effective antenna height of 6m (probably less than this in reality) and a measured antenna current of just 20mA. It works out at 315uW ERP. Based on earlier measurements by M0BMU it should now be around 60uW. I suspect he is nearer than the theory suggests. Anyway, still a long way below 1mW ERP even but getting great results with WSPR.