Polar Strap Size?

ixtee

Hey guys! I'm looking into getting a polar FT7 this week to force myself into working a bit harder. At least I think that's what I've settled on based on reviews. The only thing is, I'm not sure what size band I should be getting though. I see they have 20-30 and 30-45" options. I measured my chest right where it should be and I'm getting 31" around with no slack. I tried to do some research on how accurate those sizing options are but couldn't find anything online. From those of you who own the meter....should I be getting the XS/S or the M/XL? Kind wish they had something inbetween those!

Thanks!

** Just FYI, I'm pretty thin in my torso especially my ribcage, so I don't expect to be losing anything there. I don't expect the measurement to change.