sometimes i use morph calculators, but mainly to confirm what i calculate on my own.
clown to black pewter het clown would give 12,5% of the desired black pewter clown. also 12,5% for pastel clown and 12,5% for black pastel clown. thats quite good, also because the stuff that you produce and that doesnt look like a clown will be 100% het clown.
and yes it depends a lot on what animals you can find. if you get a black pastel het clown and a super pastel clown, for example, you can do it in one generation because the odds go up to 25% for black pewter clown. then it can really be a 2-year thing.