Ryan (and Eric) are correct. Volvic is nice because it comes from a single source and is consistent in mineral content, and can be used as-is without worrying about scale, and it should be fine for taste.
It isn't in the range of most of the ideal zones because it's a little low in the Ca and Mg cations that conventional coffee water wisdom believes is needed for tasty extractions, and is maybe just a tad high in alkalinity that could in theory dull the brightness (acidity) that you might want to bring out in some coffees.
Looking at the Volvic point on the hardness/alkalinity graph
, you'll see that you could get into the 'ideal zone' with about a 15 mg/L increase in hardness. To get, say, to Scott Rao's recommended zone (70 - 80 mg/l hardness, and 50 mg/l alkalinity), you would need a 7 - 17 mg/L hardness increase and a 12 mg/L alkalinity decrease
. You could get there by making a mix of distilled and Epsom salt that you use to dilute your Volvic.* (I'm not advocating this, because I doubt you would notice a worthwhile improvement in taste.)
I think using your tap water to bump the hardness is ill-advised, mostly because Volvic already has a bit of chloride (14 mg/L) and your tap would cause an increase in that, as well as the sulfate. Both of those might contribute to corrosion in your gorgeously restored Faema Lambro.
* If you were to make up a solution of 1 mmol/L magnesium sulfate by adding 0.26 g of Epsom salt to 1 L of water, then use 81% Volvic and 19% of this solution, you would have water with 70 ppm total hardness (33 mg/L Ca hardness and 37 mg/L Mg hardness) and 50 mg/L alkalinity. Of course, it's hard to weigh out 0.26g, so you'd probably use a concentrate - add 5.2 g of Epsom to 100 ml water, then add only 5 ml of that concentrate to your liter of distilled.
Note that your sulfate in that 1 mmol/L solution would be about 96 mg/l, and the Volvic has about 8 mg/L, so your 81/19 mix would come out to about 25 mg/L of sulfate (SO4--). Might be a downside there with respect to corrosion. Fortunately the water has good alkalinity to help offset that.