From my tap water, I too, get around the 30 - 40ppm of nitrate.
I have not tried the 50% RO / distilled water mixed with tap water as my tanks tends to have quite a few plants and by the end of the week it rare to see the nitrate above 5ppm if any at all. Plants do absorb nitrate if there is not much other fertilizer or nutrients in the water column.
Having said that, if you only have a few or no plants and therefore nitrate will be a bit too high for your stocking then the 50% mix will work nicely.
I agree with byron re to watch out for which bottled water you purchase for this, probably best to stay with RO water which is easily obtainable from most LFS at a fairly low price.