you are comparing a mountain bike with mtb bars with a dropbar bike, so as you must not have a dropbar bike, I would highly recommend riding some borrowed bikes with dropbars so that you can get some reference measurements to compare the 520 to.

dropbar handlebars are going to be more forward than mtb bars, so you cannot go with the toptube dimensions of your mountain bike.

perhaps visit some bike stores and sit on bikes, although the best would be to ride some borrowed bicycles.
If you do not have dropbar riding experience, it may be harder for you to evaluate things.

good luck