Dress Question:

I'm in the process of ordering my colored crinoline from an Etsy vendor, and she asked me a question that I'm not sure how to answer (regarding the hem of the crinoline versus the hem of the dress):

"As a general rule, I would recommend lifting the bridal slip 2" above the floor for ease of movement. Do you know how long your dress is? Or how you plan to have it hemmed in relation to your waist to floor measurement? Knowing that may help me give you a more accurate recommendation."

I'm a little under 15 weeks from the wedding, so I haven't had my dress hemmed yet.  I bought a dress at a Sample Sale last year, and it will definitely need to be hemmed (I'm only 5'3").  I haven't consulted a seamstress yet since I'm over three months off.

What's the general rule with bridal dress hemming?  How far off the ground does the dress fall?  Is waist to floor (while wearing my wedding shoes) measurement a good indicator for dress/crinoline length? 

If it helps, my dress is somewhere between a Full A-line and a Ballgown dress with a long train.

Gawd, I'm so clueless.