SPY will have to do something relatively positive today to maintain xFF. The table below shows action since April with xFF and xCF.
Note the variability in xFF sACC, LABU and TNA are negative and CH vs CL for them is negative. However, for the other guys, CL is almost trivially low. These are remarkably good numbers.
Volatility increases for xCF, but all the return numbers are consistently positive. The CH/CL advantage is also remarkable if less spectacular than xFF. The days for each are in CFF5 in the last column. xFF = 8, xCF = 17
