My experience the last few years has been running the Foss primarily during snowmelt phase. The ranges I've encountered as being run off snowmelt are from 7k to 9k on Sky guage at GB. I've discovered that above 9k (on snowmelt) the run becomes more classV. Thats generally when Rob and a few others might start to consider it fun. So for me, what I consider to be a happy medium is somewhere b/w 7,500 - 8,500 depending on whether it's going up or coming down.
For rain events look for levels on the Sky to be above 10k.
A good fall back option is the Rapid. Generally, if the Foss is high the Rapid is a good alternative. There are very few feeder lakes to the Rapid, it has a much lower elevation when considering snowpack meltoff, and so often has less water than Foss. It also stays classIV at higher flows, though the run is riddled with classV hazards.