What's the best way to interconvert between molarity and percentage? Let's say that I want to convert concentration of an acid from % to molarity.
One way I found to do this is by using the following formula:
percentage = (molarity * molar mass) / 10. 

Another way is to use an equivalency chart like this one (see bottom of page): http://www.ochemonline.com/Acid_and_Base_Concentrations

For example, to convert 38% HCl to molarity I calculated:
38% = (molarity * 36.46) / 10 and molarity = 10.42 = 10 M.
However, the equivalency chart says that 38% is actually 12.4 M.

The same discrepancy happened when I tried to convert 68% nitric acid molarity. Using the equation, I got molarity to equal 11 M, however, according to another source 68% nitric acid is closer to 15 M.

So my questions are:

1) Is there something I'm doing wrong that I get such a huge discrepancy?

2) What's the best way to figure out molarity from % and vice versa?
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!