From Finnex RAY II & FugeRAY PAR Data
Notice that if you have virtually no substrate in the tank you would get about 48 micromols of PAR, but if you have 2 inches of substrate that goes up to almost 70 micromols. Both numbers are way too much for a Walstad method tank. For that you should try to stay beween about 20 to 30 micromols of PAR.
If you can hang that light above the top of the tank, you could use the 24 inch long version, and hang it about 6-8 inches above the top of the tank, getting around 20-30 micromols.