CamBarr, the picture of your tank should not require dosing to keep parameters. what Salt are you using? Have you test your salinity? If you are dosing so much, your salinity must be really high? Also check your test kit with fresh salt mix water. If you are using the test kit properly and no problem with test kit, you should seem parameters higher than your tank water.
Hope this helps. I have been through this before.
350 ppm for calcium is okay, so I think the tank should be capable of supporting good growth now.
Instant Ocean is not a reef salt. You will be better spend your money on Reef Crystal see link.
Crystalhttp://www.bulkreefsupply.com/instant-ocean-reef-crystals-salt-mix.html
then you will not need to dose so much.
I suspect that the water volume estimate is a bit off, but I wouldn't worry much yet. Which kit are you using?
Okay, the LaMotte should be a good kit, although it's likely calibrated for fresh water. It'll be a bit off in seawater, but that's okay.