Rather than Phosphoric, try Hydrochloric acid... you can buy 5 litres from City Farmers for the same price as 250ml of Phosphoric, it's also better for the fish... and that's coming from someone that sells Phosphoric!
As far as pH meters are concerned, you get what you pay for... buy a $25 model off ebay via Hong Kong and by the time it gets here you should probably consider ordering another... so by the time the first one craps itself, the second one should be here.
The top of the range for home use is probably the Eutech/Oakton waterproof meters... about $120 - $130... from your local hydro store, or maybe Joel sells 'em as well... dunno?
I have recently started stocking a budget range ('EP' brand) of pH meters for $69.90, you may wanna try them if you can find one, the distributor assures me they have a very low return rate. It's been all good with them so far, but I haven't been selling them for long enough to give a 100% definitive answer as to their reliability.
As far as "conditioning" your top-up water, a pH of 6.0. or at least in the low 6's would be better, this would ensure all the carbonates are consumed before the water is added to your main system.
I fill an opaque (so the sunlight can work on the Chlorine) 10L watering can with tap water, then add 0.75ml of Hydrochloric to the 10L, this brings the pH of Perth tap water (I'm NOR as well) down to about 5.8 or 6.0 initially, and it's in the low 6's by the time I add it to my system 24hrs later.
Cheers.
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