1/24
Yes no one ever pointed out that 1/4 bit is actually the wrong size. 1/4 bit to half depth yields a 1/16 or slightly heavier jighead. I used a 1/4 to clean up a 1/16 mold the other day. 7/32 would probably have yielded better results. Ball end mills can be bought from any machining place. The proper way would be to find the surface of the mold by laying a piece of paper (.003") in between the mold and the bit then setting the depth for half of the bit diam and using a low speed probably around 50 rpm . The slower rpm allows the bit to easier center in the hole. good luck on your molds.
"Some days im Basstastic other days im crapptacular"