How long are you letting the cast sit before demolding?? SC305 is the SLOOOOOWWWW set stuff. It calls for 30 minute demold time, leave it for at least one hour. As modelnutz mentioned, temp is a factor. As well as something called cross sectional area. Or, in layman's terms; the thicker the piece, the better/faster is will cure out. Or, backwards... The thinner it is, the longer it will take to fully set up and become solid. Adding heat after it has set (low over time), will help speed up the curing time. Polyurethane two-part resin is an exothermic chemical reaction cure system. So, the thinner the parts, the less heat is can create internally, and that affects the cure time/firmness of the part at demold.
The mix ratio is forgiving, but with side effects, as you have seen. Too much "B", and it becomes brittle, too much "A", and it goes milky clear, and rubbery. Neat to know if that's the effect you want/need.
Get some of those medicine cups. I have tons of them, and I USE them too.
In some cases, you may need to mix up MORE than what you need to stabilize the mix ratio. Have a large mold, are a bunch of little molds, and fill those. I have a bunch of greeblie molds and thing like that that I fill when I mix too much, or need to mix more than needed for the mix ratio to be right.