Baking soda is NaHCO3. Rotalabutterfly has this in the calculator, you'll select "Dose to reach a target", and just type in what you want your KH to be. It'll tell you what your dose should be. I just buy the Arm & Hammer baking soda in the 4lbs boxes. I buy a 3 pack, it's pretty cheap, but as long as you're getting pure NaHCO3, it doesn't matter which one.
I do all my calculations based on my 5 gallon buckets, since I fill those and add them to my tanks. I dose 2.04 grams into ~4.5 gallons of RODI to get a KH of 4, for example. You'll only use it at water changes, as with Equilibrium. Add it to your water though BEFORE adding it to the tank, otherwise you could cause osmotic shock in your livestock, leading to death. This would be a much greater concern doing a water change with pure RODI, but I still would have your water pre-mixed before adding.
If that 700 gph rating is with media, then you should be alright. I have a feeling though that they rate the gph based on an empty canister. With the canister loaded, that will be significantly lower. We typically aim for 10x water turnover per hour, so that's close. I won't say it'll be a problem, but you may want to setup a second filter to make up the difference. Or, just wait and see how it goes.