The magic number to remember is 0.67
You still add plaster until the "mountains" form and you get the "cracked desert 3 days after a storm" texture. This is not an alternate method; it's simple preparation for any method that involves moulding plaster or pottery plaster. You can verify this calculation using a graduated bucket and noting the volume difference before and after adding plaster until the "cracked desert" happens.
Visual approximations of volume can be difficult to make. Being exact is easy. Minimizing leftover plaster means less to end up solidifying in the sink and plumbing (where it shouldn't be anyway) or in the trash can (where an accumulation would mean a heavy burden for whoever takes out the trash).