Ujm, according to the 2013 pew study on American jewry, 10- 19% of reform Jews (the variation is due to a difference between those who identify with judaism as their religion vs their culture) say that their mother is not Jewish.
Conversions aren’t that common, with the JTA claiming that the reform HUC has logged 1000 participants in conversion classes in the year 2013 – there are 8 million jews in America, so this is a tiny percentage. Conservative conversion is even less common.
When we eliminate the “did the mother convert” element, (which obviously needs to be inspected before recognizing someone as Jewish), we’re left with 80% of reform jews who are most likely halachikally Jewish. If we say that of those 80%, the mothers themselves were the product of perhaps a jewish father and non jewish mother, we’d fall back on intermarriage statistics from 30ish years ago(a generation) which was only 15% across the board, higher among reform but not by much, according to the north American jewish data bank (1982-87).
Worst case scenario would be that 65% of reform jews are halachikally Jewish.