What if I don't change my Master Cylinder on a disc conversion?

You could do the math. Typically the pistons retract maybe .003 inch, maybe .005 if the discs have a little run-out. And typically the 73-up pistons are 2 inchers. So; using the pi.r sqd to find area and then multiplying by the travel, .005inch and the doubling it for both sides, you get .0314 cubic inch. to convert to CCs multiply by 2.54 cubed. And that, my friends, gets you about 1/2 CC, total. I love math. Its a non-issue. just remove the residual, and check the level now and then.
- And if you do the math on the rears, with typically poorly adjusted mechanisms;lets say a 7/8 bore and at least 1/8 inch of travel per side(probably more) you get about 2.5 CCs total.
-I love math.
-Now, if you have the KH 4-piston units; I cant remember the piston size, so Imma gonna guess about 1.125 inchers. And there are 8 all together. And they typically retract maybe .004. Doing the math gets you, again, about 1/2 CC total.
-Did I mention I love math?
-Not another word about running out of fluid on the first application!
-Somebody want to do the math on how many CCs are in the MC? Im gonna estimate this one;LxWxHx2.54cubed= about 140 plus CCs per small chamber. So if she starts sucking air at 80% empty, that would be a lotta strokes.
-Bear in mind, that if you remove the calipers from their brackets, and stroke the pedal so that the entire chamber in the M/C is flushed out to the pistons, well thats some new math, which wouldnt really be relevent, so Im not going to bother.

Unfortunately you are not solving the right problem. I'm not sure what Scarebird calipers are specified, but it does not really matter. The disc master cylinder volume of brake fluid is the {area of all the pistons x [thickness of the brake pads + (max rotor thickness - the minimum rotor thickness)]} X about 1.5 factor of safety. I've seen rotors that have been chewed into the vents and still have fluid in the master cylinder. Maybe your average Canadian is smarter than the people we have to design for in the US. I'm also sure this is a simplification of the problem. You can do the math, I'll trust the engineers at Chrysler to have done the right thing.