If that's your actual size, then you know your measurements for Underworks; (34+37)/2=35.5 would put you squarely in a medium for the Tri-top. But how confident are you that that's your "true" size? I thought that was my bra size, until I took measurements specifically for getting a binder, when I discovered I was way off; (30+34)/2=32 puts me at the lower end of small for a Tri-Top, and I find an x-small fits better. Not surprisingly, the bras that sat around my closet not getting worn (I don't wear bras) were the wrong size anyway, and I just didn't care to figure it out until I was binder-shopping! So, anyway, why not just follow their instructions and take the measurements yourself, then decide which size you want based on how close you are to the cutoff between sizes?