This will give you say a perfect 3 8 caulk line with an irregular wall to counter gap.
Big gap between counter and wall.
It s supposed to be behind the counters and cabinets.
The line will continue maintaining caulk gap regardless of the gap of the wall to counter.
Tiles are started 1 8 off the counter leaving gap for caulk.
If the countertop isn t level slide shims between the countertop and the support struts of the cabinet frame to level the countertop.
If it s not too wide i d line the back of the counter with crown molding or wall trim of some kind paint it with a high gloss paint so that it s washable.
Remove the excess countertop up to the pencil line with a belt sander and a coarse 60 grit sanding belt.
Measure the depth of your counter and cut the cover if necessary.
An obvious gap between a countertop and backsplash is unattractive and it can allow water to seep through.
2 phillips bit a flat pry bar wood shims and a hammer.
If there is say an inch to inch and one half overhang across the front of your slab maybe the slab could be slid back say one half inch closer to the wall closing the gap enough that when you add a backsplash you could fill in the difference with foam backer stuffed into the space between back of countertop and wall and then use flexible caulk to close the gap would depend on how thick your backsplash is as to whether this might be a viable fix.
If the gap cover is shorter than the depth of your stove leave a gap between the wall and the cover.
If the gap is too wide to seal with silicone narrow trim molding can solve the problem.
Your tile person will take the largest gap and run a tape line using that as the thickness or the caulk.
Measure from the edge of the counter to the back of the stove to determine the correct size for the gap cover.
The fix is to remove those counters and actually fix the wall.
If you can t do that then you are in over your head with this house and it will do noting but bleed you dry paying for people to do work for you.
Set your compass to fit the tip in the widest gap between the countertop and the wall.