My advice is always to start out simple. To your 6.5 gal of mash water add about 3/4 tsp of calcium chloride. That's all you really need to do but one question is clearly left unanswered and that is 'what about sulfate?' If you know, from your other beers, that you like the effects of sulfate then use half gypsum and half calcium chloride. In either case after the beer is brewed experiment with adding additional amounts of gypsum and/or calcium chloride in the glass to find out where your sulfate sweet spot is.
Dark beers are always a bit tricky because of the acidity of the dark malt and you have no alkalinity to absorb any extra acid. Indications are that you will probably be OK and will probably have a mash pH close to 5.4. You are reasonably secure here because you have a relatively large amount of base malt to absorb any extra acid. Making certain assumptions about how your malts resemble the malts that I and Kai Troester have measured in the past I calculate a pH of 5.39. If the special B turns out to be twice as acidic as I have assumed then that estimate drops to 5.36 - the rest of the malts are buffering for you. I have assumed that your base malt has a DI mash pH of 5.62. I have not measured MO but it is reported to have a DI mash pH that low. Should the malt you actually use have a higher DI mash pH, e.g. 5.72 then that will pull your mash pH higher to 5.45. Still not bad at all. Nonetheless, especially in doing dark beers, it is always best to use a pH meter to find the pH of a test mash because of the uncertainties in the properties of the relatively large proportions of high colored malts used.
As for calculators: they abound. I'd say to get copies of several and compare them. Use the one you are comfortable with. Comfort usually means that you can use the thing with facility and that it give acceptably good pH predictions for the kinds of beer you brew. In order to know whether it does or not, of course, you will have to obtain a pH meter and compare predictions to actual measurements. If you are going to be a serious brewer a pH meter is in your future.