Hi!
Most of the calcium comes actually with unfiltered water and large amounts of pellets but not from veg. Please also be aware that too little calcium in the diet can also cause calcium pees and it can be as damaging for the health in the long term as too much calcium. What you are aiming for the is the correct ratio for your location.
Since calcium comes with all food/water groups, it is impossible to fully measure and compute the exact range. Most diets only concentrate on the veg, which make just 15% of the daily food intake and ignoring the rest, which means that they only work for a limited number of people living in a comparable situation but not the others who have different parameters.
Stopping any veg won't do anything in terms of calcium pees. Please be aware that excreting any excess calcium via the pee is a natural and health process in a guinea pig's body and nothing to worry about.
If you have calcium pees nearly every day, then they are generally an indication that the balance is not right, whether you are too high or too low of the ideal band or that something in the calcium absorption process has suddenly changed. It can in some cases also be the very first symptom of a developing cystitis, especially with a sterile interstitial one, which will develop with the more widely known symptoms over the next 5-7 days.
The calcium absorption process is a very complex one and not a quick one; it takes several weeks for any dietary changes to filter through the body. You also have to be aware that diet related stones are only one aspect and unless the diet is really grossly overladen with calcium or the calcium balance in the diet is just wrong, there are normally several other factors in play, like a genetic disposition, something in the absorption process going wrong and a bladder stone piggy not being a great drinker.
The latter you can't change but you can give a little more high veg to those piggies in order to help keeping their bladders flushed better with some daily stronger pees. Please also be aware that when you serve more fluid in edible form, piggies that are good drinkers will simply drink less water because they do not need more fluid. The more veg fluid trick only really works for the piggies that are not naturally good drinkers. Like with humans, individual fluid intake varies massively from person to person. Just because somebody is not a good drinker doesn't mean that they are dehydrated.
Diet is the only factor that we can influence, which is why it so difficult to prevent stones altogether; but on the whole it is very effective. I have haven't had any new stones since 2013 after I started filtering the water and reducing the pellet amount considerably. The spate of stones was a result of me experimenting with the diet and getting the balance just wrong for a while. Practical experiences like these have gone into our diet guidem which on the whole has stood the test of time although we are of course updating constantly and working in any insights and practical experiences that have stood the test of time. Filtering water is more crucial in any hard water areas like most of the UK but it can also help with mineral rich water that can also contribute to the formation of stones. Most stones are actually not calcium but either mostly carbohydrate or (more rarely) oxalate.
Healthy piggies without any urinary tract issues should be kept on a normal general diet with a mix of high fluid content veg and some higher calcium and other trace elements green leafy herbs or veg (the two are usually linked) that take the place of the herbal forage with which guinea pigs would supplement the mainly grass and hay based diet they have evolved on. Just because you worry about the potential of stones shouldn't mean that you end up erring too much on the other side.
Please take the time to read this guide here. You may find it helpful as it tackles the diet as a whole but looks at all food groups in practical detail, too:
Long Term Balanced General And Special Needs Guinea Pig Diets
I hope that this helps you?