how to decide on a width and offset for your wheels?

How would you determine what width and offset you need to achieve "flush" on any particular platform. In my head it seems like you would measure from the face of the hub to the edge of the fender. Double that distance to get your width. Of course this would only work with "0" offset. This also does not factor a fender roll/pull. Another complication is mount depth(clearance from the mounting surface of the wheel in) and using spacers as a solution. 
Sorry for my noob incompetence, but these all strike me as factors in determining the proper offset.

Before anyone says it, I did read the offset sticky. These are questions I had after reading that. If this has already been answered please post the link, because I couldn't find it.

id guess..  9 inch wide is a must.. 25offset or lower is a must..    what kind of car? if its a rwd sports car, 17x9 +15 is a minimum for flush on a 240sx. A 4 door sedan like most VIP cars wont look good in that size, i think 18 is a minimum. 

RWD cars usually require aggressive sizing like that.

Honestly I am not sure ... I had always compared how others fit wheels on there car .. and 
 
my gs was different than others , I was tucking 19x10 +22 in the rear .. but for flush 19x10.5 +17 on bags ..  lots of things to consider .. 
 
what car do you have ?  comparing or searching what others have done is a great way

I thought there might be a way to tell on any car. There is bound to be a process to calculating fitment beyond trial and error. I don't have the car in question yet. Just curious for future referance. I didn't want to ask someone what specs were on their wheels and comeoff as a biter. But I guess in this form of the scene people are actually helpful. Doesn't hurt to ask.
The wheels and bags probably wouldn't come all at once, so I would need a wheel aggressive enough to pull off on bags, but conservative enough to roll on a regular drop without looking like a douche. Again that is probably a misconception that I am carrying over from the Honda world (lots of JDM snob baggage). I will definately get a quality set of wheels and hope for respect

Take a stick. Tape it to your fender.

Measure from 1/4" away from your strut, to the stick. Round down to the nearest inch and remove another half inch. The reason: wheel width is bead to bead, there's usually about an inch of wheel past the beads, in total. So there's your width.

Measure from the face of the hub to the imaginary center of this 'width, in millimeters. There's your offset.

Oh, and do all that with the suspension set at the camber angle you'll be running at.

That's what I'm talkin about!
So if the hub is 10mm past the "center point" I would need a +10 offset for them to be flush? If that's the case, I completely get it. You answered my question exactly. Care to tutor me for chemistry? LOL
When are spacers necessary? I would imagine they wouldn't be with a properly fitting wheel...

Bingo!

Unfortunately, I'm mathematically impaired and nearly flunked chem lol

And also, you're correct about the spacers. When you're bang on with the offset and disk height (for brake clearance), spacers won't be necessary. That's one of the benefits of getting a set made to order.

the other way to do it is search in google for an offset calculator.
you can put in your stock wheels then the new width/offset of your new wheels and it will tell you how much further they will stick out then your stock ones so you can measure it like that.