Yes, there is a rule-of-thumb for determining shear walls and how wide windows should be to have the posts between the windows transfer the loads adequately to the foundation.
You're in one of the highest rated seismic areas in the U.S. Critical loads are moving the building up-down and side-to-side. We try to tie the roof to the walls and the walls to the foundation for vertical and horizontal loads.
I don't know the width of your house (I'll explain why that's important later), but as a rule-of-thumb, for seismic loads, you need a minimum of 4' of shearwall at each corner (before any windows) and you want no more than 60% of the length of the wall for window / door area. This allows you about 40% of the wall to tie to the roof and foundation.
For your 25' long wall, without structural calculations, I'd use :
25' x 40% = 10' of shear wall (Sorry, I know you want about 20' of window area.)
As a rule-of-thumb, the house length to width needs to be no greater than 1:3. So, if the 25' is the width, then the length should not be greater than 75'. Likewise, if the length is 25', then the width will be no greater than 25' (1:1) and no less than 8'-4"...you don't want it too skinny either.
Now the width of windows: As a rule-of-thumb, the width of each window should not be greater than twice the height of the foundation wall (stem wall plus depth of footing). So, if the stem wall is 18" high and the footing is 6" thick, then the windows should not be any wider than 4'. (As a rule-of-thumb, loads transfer from the wood posts between the windows to the concrete foundation at a 45 degree angle. So you don't want to be further apart than what can be transferred through the foundation to the ground.)
Yes, you are correct, Simpson has some great holddowns and pre-made shearwalls that we often use for garages because the garage doors exceed the 40-60% rule-of-thumb. However, they're expensive and you'll need to calculate the tributary loads. Keeping your house fairly rectangular, installing 4' shearwalls at all corners and keeping window widths about twice the depth of the foundation, you'll be safe.
By the way, use 1/2" minimum plywood (not OSB board for wall and roof sheathing) and nail at all supports with 10d at 4" o.c. at all supports...roof and wall. (If you nail any closer, you'll split the studs and have no resistance.) This all assumes the shearwalls are no less than about 1:2. If you have a vaulted 16' high wall, then the corner shearwall needs to be about 8' wide.
If these rules-of-thumbs don't work for you, you'll need to get it calculated. I'd get a hold of a structural engineer (NOT a civil engineer) in your area. Tell him you just want the calculations, not the plans. It'll cost you about 4 hours of his time (assuming he's got the software) and you can install it as per his design.
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Best Answer
The right way to fix this is to cut out the spray foam insulation and move the window into proper position so that the two units align on a single plane. This isn't as difficult as it sounds. You can pilot and countersink some construction screws through the window frame at an upward angle into the framing to pull it outward if necessary. Work slowly and use caution and the window will move easily due to flex in the mounting flange. Be sure to keep shims in place to prevent the frame from bowing upward.
It looks like you have screws running through the window frame into the header. Many window manufacturers specifically prohibit this, as it increases the chance that settling will bow or jamb the window. I suggest removing them and reviewing the installation instructions. Andersen windows, for example, call for nailing through the outer flanges only (except where special clips are provided). (Correction: frame fastening is allowed, but is only done through the sides of the window.)
Otherwise, you'll have to use a jointer, powered hand planer, or sander to shape your extension jamb to the window. I wouldn't try to remove material from the window itself. You'd do this by holding the extension in place with the same gap to the window frame at each end. Measure that gap, and there you have the amount of wood that you need to remove at the center of the extension jamb, tapered evenly to the ends. How you remove the material depends on the tools you have on hand and your skill set.