My advice for you is to follow the chart of Guinea Lynx website; it is an excellent website. There are a lot of information and charts with lists of vegs and their amount of calcium/phosphorus/vit C. There is also a calculator online (same website). It is easy to check the daily meal, if it contains too much calcium or even too much phosphorus, which is bad as well. Keeping the ratio of these two minerals at a certain level is very important and I noticed that those residuals of calcium appear in my piggies' wee just when that ratio was wrong the day before...
Those pages will explain the subject better than me (for me explaining in english is difficult...

).
Of course you don't have to go crazy over those charts... it is just for learning something more (and trying to do some good prevention of urinary common troubles in piggies)