Please note:

Silicone Lid diameter is between 80 - 95mm (sizes are 80, 83, 86, 89, 92, 95mm). You'll see a good leak-proof fit if the mug’s notched area is anywhere between 79mm - 94mm.

A few things to watch for:

1) The notched area should be as round as possible, although the flexible silicone helps compensate for some inconsistencies.

2) We recommend the distance from the lip (top of the mug) to the notch should at least be 15mm, ideally between 20 -22mm so the liquid can build “flowing momentum” after passing the notched area. And we recommend 1-2mm notch depth & around 4mm width(please see diagram below)