Here's a good gearing calculator: http://software.bareknucklebrigade.com/

Depending on terrain and fitness level, mid-60s to low 70s is a good range for gearing. If you're confident, start with a 70ish gear.

52x15 = 91 gear inches = OW.


FYI:
A change of one tooth in the rear cog is equivalent to two chainring teeth.

For your conversion, the easiest choice is to swap out the larger front chainring for the smaller one.


Choosing a more efficient gear can result in a better, faster, and less injury-prone ride. (Translation: nothing macho about the big ring/high gear, and it won't necessarily get you from A to B faster than an efficient cyclist.)

For general riding on a conversion, the middle or second chainring makes more sense than the larger one: i.e., 42x15 = 75 gear inches. Try that first and then see if you want a larger chainring.