When I first started out designing, I didn't understand the concept of designing a bra around a wire, most of my designing was done using pattern blocks that were already available and I just altered maybe the style line or fabrics but never the shape of the bra where the underwire sat.
It wasn't until I started my own brand that I began to understand the true importance of designing the bra around the wire, it was a concept I hadn't grasped before, yet it's the first thing I start with.
You start with finding the balance point of the wire from there you can draft your cradle pattern.
The balance point is the deepest part of the wire. If you draw an 'x' axis and 'y' axis and put the wire on it, the deepest part is the balance point, with the CF of the wire (the shorter part) against the 'y' axis.
If you're new to finding the balance point, I'd draw a line on the wire with permanent pen so you can locate it easily throughout your design. If you are designing a bra with a cradle, where the centre front piece joins the wing, the join of those pieces should be where the balance point of the wire is.
Draw around the wire (pictured is a balconette wire), then draw in a new line 15mm away from the original wire.
At the underarm the cradle should sit anywhere from 15mm away from the wire to approx 25mm though I have known some people's patterns to sit further out if they are designing for plus sizes. Having a wire to splay out is so when the bra is worn the wire is sitting at the root of the breast not on it. I usually start my patterns at 15mm and alter it after fit. If there was no splay in the wire then the support wouldn't be there, the wire would be just sitting there and not have any tension on it.
Some pattern makers splay the wire at the centre front (usually by 5mm) and the wing by 15mm, by personal choice when I'm designing for my brand Vanjo, I tend to only splay at the side, as my core customer are women with big cup sizes and small backs, so I use quite firm wires and find working with a smaller backs this worked best for me.
Another thing to take into consideration, is wire play, this is the amount left at the end of the wires. This is not seam allowance, this is the finished amount left. Wires need some room to move, with no wire play (room at the end of the wire) what is likely to occur is that the wire will come through the wire casing after repeated wear, as it's pushed up against the fabric with no wear (pun intended) to go.
On average leave 5mm at each end of the wire and approx 3mm where the bar tacking will go. So all in all draw in approx 8mm at the end of each wire.
Depending what type of centre front you are looking to have, to draft the pattern I would draw the centre front piece at a 90 degree angle coming off the wire. All these are suggestions are starting points, and can be altered after fits.