What's the best way to interconvert between molarity and percentage?
Thanks to LDC3's comments, I understand that the best way to convert from % to Molarity is as follows:

1) multiply density by volume at hand to get weight

2) solve ratio for x, where x is grams of solute (in volume at hand)

(given% / 100%) = (x / weight from step 1)

3) multiply x by molar mass to get moles (in volume at hand)

4) if needed, multiply resulting moles/volume in hand to get Molarity

Example: Let's say I have 100 mL of 38% HCl.

1) 1.19 g/ml * 100 mL = 119 g

2) (38%/100%) = (x/119) where x = 45.22 g

3) 45.22 g * 1mol/36.46g = 1.240 moles

4) 1.240 mol/100 ml = 12.40 mol / 1000 mL = 12.4 M

Now, the result matches the equivalency chart!