Don't bother to try taking a casting with plaster that shrinks too. Go for the more expensive modeling clay (may be this is good for you http://www.columbusclay.com/Modeling_Clay.htm) or do a layer of latex (often used for mask making) as a first layer and then use the shrinkable stuff as the next.

X2 or 3 or 4 or whatever for the shrinkage of a casting though

http://www.shelmetcastings.com/shrinkage-allowance-for-metal-casting.html

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!