Brand spanking new and terrified

Hi, welcome to BT and triathlon! No need to be terrified all of it is just one step or stroke at a time.

Standover height, or anything involving formulas, isn't a very good way to size a bike for yourself ... what you need to do, if possible, is get a bike fitting at a good local bike or tri store. Those personal measurements will indicate what size, makes and models of bike will suit you best.

I'd encourage you to post your question in Triathlon Talk.

As Yanti said stand over height is not a good indicator.  Go to a bike store and get fitted. They at least can give you an idea of what size you really need.  Welcome to BT

Welcome to the multisport world!  

Before you invest too much time and money in a bicycle, do you have a bicycle and what kind of riding do you expect to do?  

Road and tri-bikes are efficient, with light weight frames, wheels, and tires.  Wonderful when you want to go fast or go far or both.  But...the riding position might not be comfortable (physically or psychologically) at first and you might spend a lot of time dodging glass and fixing flat tires.  

Mountain bikes have heavy duty frames and heavy duty tires.  The ride can be a bit "buzzy" if you still have knobby tires, but you ride in a more upright position so it may be easier to see and be seen.  Not as efficient as a road bike, but you won't spend as much time dodging glass or potholes.  The more upright riding position is easier on your hands, back, and neck, but harder on your bottom.  

Hybrid/City/Fitness bikes are a cross of the two.  If you are only going to have one bike, this might be a decent choice.  You can easily ride around town and would have no problems taking it on a 50 mile ride (after building up to that distance).  You can get combo pedals and ride in bike shoes or regular shoes.  

Regardless of the bike, the seat position should be high enough that your leg is almost straight when you put your foot on the pedal at the bottom of the stroke.  You'll notice that this means you can't sit on the seat and put your feet on the ground.  If there isn't enough clearance between the top tube and your crotch, you'll have to lean the bike to the side (think of standing with your left foot on the ground, right foot on the pedal at the bottom of the stroke, and the bike leaned towards the left as much as necessary.  There is some safety element that you'll want to be comfortable straddling the bike all the time and in a hurry, so don't get a bike too big.