How Do I Convert Thousand Cubic Ft. To Gallons ?

1 cubic foot is 7 37/77 gallons (exactly). That's about 7.48052 gallons/ft³
100 cubic feet is 748 4/77 gallons (exactly). That's about 748.052 gal/ccf
1000 cubic feet is 7480 40/77 gallons (exactly). That's about 7480.52 gal/mcf
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I'm not quite sure where the 1.21 factor came from. 1/748.052 ≈ .0013368
To convert from ccf to mgal, you would divide by 1.34
To convert from mcf to mgal, you would divide by .134

Your 1.21 number looks like it has a built-in "error" (or safety?) of 10%. The corresponding number for mcf would likely be 0.121.

Example
1 ccf ≈ 1/1.34 mgal = .746 mgal    (compare to the actual value of .748)
1 mcf ≈ 1/.134 mgal = 7.46 mgal    (compare to the actual value of 7.48)

Your conversion...
1 ccf = 1/1.21 = .826 mgal
1 mcf = 1/.121 = 8.26 mgal    (this is about 10.4% higher than the actual value of 7.48 mgal)