I slightly over fill the flask.
I use the convex edge of a trowel to scrape away the extra
investment. Scrape the investment until the edge of the trowel
scrapes on the bottom edge of the flask.
This process forms a slight dome in the investment which distributes
the vacuum to a better degree.
The outside edge of the investment will press against the silicone
pad whenpulling a vacuum. The investment cannot slide out of the
If you are casting sterling check out the paper I wrote and posted
to the TIPS FROM THE JEWELERS BENCH on fire scale prevention during
vacuum casting. The process works and does not cost and will save a
large amount of work requires to remove fire scale.