The alloy content with antimony has a little understood factor. Antimony is the only element that gets larger as it cools. This is because the crystalline structure created by cooling is more rigid. Antimony is used in linotype printing to sharpen the edges of the letters and make the printing more distinct. Alloy with large amounts of antimony get larger as they cool.
The heat of the mold is also a factor. Each degree of heat using meehanite alloy will increase diameter .0001 of an inch. Thus a mold will increase diameter as it heats. Negible factor unless your pot swings in a great range of heat while you are using it.
The mold maker for my current molds recommends 775 degrees for Linotype alloy and 845 degrees for 20:1 Pb/Sn alloy and that is what I run. He knows and built them that way.
Be consistent for your heat settings and develop a rhythm in casting that keeps the pot at a constant temperature.
Farm boy from Illinois, living in the magical Pacific Northwest