I am wanting to get Sitka 90% or similar and I was wondering how cold you could comfortably wear a softshell while moving. I have a Cabela's insulator jacket or patagonia puff vest for stationary insulation.

Right now I have a couple lightweight Icebreaker base layers and I have a Sitka Celcius vest on the way. With those 2 layers under a 90% I am thinking I should be good down to 15-20 degree wind-chill. Does that sound about right or am I off base?

that sounds about right to me, but it also depends upon your definition of "moving"...

in other words, there's a difference between hauling up a mountain and slinking slowly through a river bottom...

IME, if you stop for any length of time at 15 degrees, you'd want more than a baselayer, vest, and softshell. But, it'd be fine for moving along in the mountains with pauses.

There are all manner of "softshells" from heavies with fleece laminated to them, to thin varieties with no insulation and little wind-blocking ability.

"Softshell" is a BIG category.

For on the move hunting I wear a very thin Mountain Hardwear "Offwidth" jacket... it's perfect over a midweight zip t-neck in typical Novemeber temps. A midweight or heavy softshell is too heavy in temps from 0-45* for me.

I wore my Offwidth and midweight zip t-neck, while moving, a -23*F last hunting season and was fine. If you stop, you're going to get cold though!

That temp range would be a little optimistic for me unless I was at "Full Throttle". But then again, I tend to run a little colder than most so YMMV.

You will get mixed reviews of soft shell garments, but I have been using a 90% jacket for several years now and can't seem to leave it behind. In fact, I would have to say it is my most used outer layer for hunting over that period of time.

I want to get a light fleece like an R1 to go under the 90% as well so that should be plenty for where I hunt I imagine. I wish I could find a store around that carried the Sitka stuff so I could try one on. My Cabelas here in Kansas City doesn't carry any in the store.

That sounds pretty close, if you are moving fairly slow. If you hiking at a good pace, I would guess you'd be getting overheated.

The other day were hunting in single digit weather and I had on an Icebreaker base, Sitka Core top, light fleece pullover, and the Sitka Celsius vest and I was a little warm for moving hard, but just about right for an almost still hunting pace. Opening all the zippers did help a good bit when we were moving hard through about 5 inches of old snow and 2-3 inches of fresh powder.

WIth the venting of the 905 I could see that set up working very well. Heck, I use a baselayer, R1 fleece, and a puffbass as needed for the same types of conditions. IMO, softshell jackets just make too much sense for hunting and I definitely plan on adding one to the mix next season.