The above posts are accurate in that Foxton flow can be estimated by the Pine gage. However, I spoke to the actual operator of that gage (a guy in Lakewood office of USGS) about 2-3 years ago, and he admitted that they did not really have the gage dialed in yet. Note that there are only 4 years of record, so the bugs on the stage vs. flow correlation are still being worked out. He also indicated that the gage itself was at a less than desirable location, and subject to malfunction. Therefore, I would not recommend solely relying on this gage.
When I ran Foxton frequently, I would base my decision on examination of the Bailey/Grant gages, the Pine gage (not shown on the Phlows page), and the Waterton gage. If all three seem to point to the fact that there is water, I would go. My personal low was 150 which is enough to float all areas, but that would require leaving some plastic. I caught it once last summer at 600 when visiting the FR, and it was awesome then.