I agree with the wrong way round aspect, but my understanding is KH is what helps buffer against pH swings, not GH. GH does measure carbonate, but also several other minerals, so it's more accurate to consider KH when worrying about pH swings. Plus gemelee wants soft water...
gemelee - I wouldn't worry about a few degrees of KH. Every time I've tested Hams tap water the GH has been zero (as far as an API test kit can discern), so if your pH is stable then just keep doing what you're doing. If it isn't, then try raising your KH a few degrees. I run some lime chips in my filters to bump up the KH. There's a landscape supplies place in Frankton that'll sell you a few handfuls for a dollar. It takes a bit of trial an error to figure out how much you need, but they last for ages and ages. If lime chip don't raise it enough you can try crushed shells, which have more surface area. The same place sells them too.