Jon, maybe you are losing magnesium faster than the lotion can provide. If you are not eating magnesium rich foods, or you use some drug(s) that deplete this mineral, you may need to also use an oral supplement with the topical lotion. Diabetics lose lots of magnesium thru the urine on a daily basis.

One SlowMag twice a day, may help.
Do you have any muscle tightness anywhere else? That would suggest a magnesium shortage.

But if this is only in your legs...that may suggest a circulation issue.

Heavy caffeine consumption, diuretics for blood pressure, alcohol, steroids, some chronic antibiotic use, and chronic use of acid blocking drugs may lower magnesium in the body.

Aberrations of calcium levels also can cause severe muscle cramping. This can be tested for. Low potassium can occur with some kidney problems. Diltiazem can cause muscle cramps, and kidney problems can cause either low or high potassium, so that should be tested too.

Problems with your arches and feet in general can strain the calf muscles. If your cramping is only after lots of walking, the problem may be mechanical in your feet. If so orthotics may help.