I am currently running a 20mm offset on one of my bikes and it's not quite enough. Question is this - will going to a 25mm offset get me exactly 5mm more or is there some weird math involved based on the seat height and ST angle?

There is no complication with the actual fore-aft setting or its effect on saddle height, but usually when you move the saddle fore-aft it changes your leg reach to the pedals. So you generally do need to adjust saddle height. If I remember correctly, the quick and dirty is this: for every 10 mm you go forward (backward) add 1 mm extra and raise (lower) the saddle 3 mm. The extra horizontal movement accounts for the backward slant of the seat tube. Sounds crazy but it works.

BTW, seatpost set back and saddle setback are not quite the same thing. I was a little confused by the differences between your thread title and your explanation. I think you mean saddle setback not seatpost setback.

I already have the fit numbers from the Cervelo I am trying to transfer to the Scott. The problem is that with the 20mm offset seatpost on the Scott, I can't get the needed 77mm setback on the saddle with the saddle being pushed all the way back.

I am seeing the seatpost referred to as both setback and offset.

So, am I correct in saying that per given saddle height and position relative to the clamp going from a 20mm offset seatpost to a 30mm offset seatpost will net an additional 10mm of saddle setback?

Yes, the 30 mm setback post will provide an additional 10 mm of setback if you choose to use it all. The need for that much setback suggests you have very long femurs or else your frame has a very upright seat tube. Does that make sense to you?

Yep. I am not sure about the long femurs vs overall leg length, but the fitter did say that my legs were long for my height. The Cervelo ST is at 73*, I think Scott is either 74 or 74.5 so a little steeper and based on a quick google seach 1* is about 10mm difference.

I knew before I scrolled past the first post that you'd be the first to answer the question! Rock On!

It doesn't always happen, but there is something so comfortable about an orderly universe.

Depends on whether or not basic geometry and trigonometry are categorized as weird math.