This discrepancy should have been caught when the countertop guys templated the countertop. Are your walls that messed up, or is it because of a flawed cabinet installation? Regardless, a 1/2" gap is ridiculous. You can have a new top properly fitted. Bingo, problem solved. Is there a backsplash to be installed? That can hide a gap, with the exception of the gap at the end of the countertop. Depends on if that edge will be visible or not. Everything else is a compromise of sorts in that you can reduce the gap in the back but you overhang in front. You can slide it back but then your cabinet overhang at the front of the cabinets will be skewed. You can scribe the countertop as is, slide the entire thing back 1/2" to close the gap on the right. That'll give you even reveal on the front overhang, but it'll be 1/2" less front cabinet overhang than you have now. Or you can do a fraction of that 1/2". Slide it back 1/4" for a 1/4" gap on the back left, and lose 1/4" of overhang at the front of the cabinets. You can notch the drywall on the back right wall and slide the top back. Same effect as the previous recommendation, but you're notching the drywall instead of scribing the countertop. Another alternative is to use mud to float out the back wall. Essentially you're making the back wall "thicker" to bring the wall forward to meet the countertop. It'd have to be done in several steps, and you'd have to feather the half-inch thickness down over the run of the wall. But that also depends on windows on the wall and how the existing window trim will be affected. There are fixes and there are band-aid repairs. Mongo

On the other sidde of your question, most tile used in backsplashes is 1/4" or 3/8" thick, plus you have the thinset behind that. You can probably allow 3/8-1/2" for your splash, but I agree that 1/4" variations are probably more than norm that unusual. Hope you love your marble. I thought I was going to have to tell you that I had the first stain on my marble, but I think they were actually on the sealer. They came right off with some cleaner add I will reseal the counters soon -- it's about time anyway. I think you are more likely to love them for several years and convince someone else seeing them in a used state that marble isn't so scary that to lose a sale. Worst case, someone wants to replace the counters -- or get Dry Treat applied. ;-) For now, they are yours to enjoy. I like your slabs. The speak to me -- kind of like mine did. Mine had dark lines and squiggles against a background that reminded me of da Vinci sketches on old paper. My son has done reports on da Vinci for school and we have enjoyed talking about his incredible visions and ideas -- and laughed about the line in "Ever After" where da Vinci takes a locked door off the hings and tells them that he shall forever be remembered as the man who opened a door. I think that set me up to love my slab, but yours has a wispy quality that is unusual and I like that too. Enjoy your counters, and if you have any concerns 3 years from now -- well, no certainty that I'll still be here or have the same email address, but I'd be happy to help you get them ready for a resale over a cup of coffee and talk about what your next counters will be (these are it for me -- DH says he's going out of here in a box -- LOL) Love your marble!

I would say the standard is "no gap" that is visible to the eye. If there is gap due to defect of the wall (it is not square), then the countertop should have been cut to accommodate that. I would not be comfortable with caulking of that size. Only other way to mitigate it is to have a backsplash made out of the same countertop material. that way, it would be thick enough to cover the gap, but have something to support it other than the caulking.

You could nail a 2x4 horizontally across the studs, cover with cement board and tile on the front, add a wood or tile top ledge and use it for spices. If 4.5cm is too narrow, add a second 2x4 to make the shelf 3.5" (8.89cm) deep. Then tile the backsplash above the ledge. I think you can get marble thresholds at the big box stores, to use for the top ledge (if marble would match the kitchen). If floor space permits, you could pull the cabinets out to make the shelf/ledge even deeper. I had a spice shelf like that on top of the typical prefab coved laminate backsplash, in a kitchen 20 years back. What kind of counter material are you using? Lowe's 2" and 4" threshold tile 4" marble side splashes