When calculating for an actual emergency (in which you WILL be at least a little stressed) you should use a much higher SAC than your normal rate for calculation purposes. I’d say if your normal SAC is .75 you should figure on your emergency SAC being at least 1.0 or even 1.5 at least until you get to the safety stop where you may have time to calm down as you are now "almost home"
In practical terms, that means figure on needing 1.3 to 2.0 times as much emergency reserve as you calculated above. An AL30 or AL40 will give you that extra margin of safety you will want just in case of heavy breathing under stress.
So if you do choose to solo dive with "just" a pony instead of a fully redundant gas supply such as isolation manifold doubles or 2 full size side mount tanks, I would recommend slinging a larger pony such as a 30 or 40 cf tank. They are only marginally more expensive than a 19 and not that much more weight or drag to carry and have 1.5 to 2 times the gas in case you need it.