The sizing is taken from Pd-extended which largely had tested Tcl font rendering across all platforms. With the same font, the patches should render the same size most everywhere, as shown with our testing. I would still suggest not relying on *exact* pixel sizes and give yourself a little padding here and there to be safe.
In some ways, this was really not a fun problem as people moving from Pd-extended had broken sizing and people with tight GUIs like yourself have broken patches, but it should be *much* easier to resolve the sizing in the future. OTOH maybe it's worth talking about a different sizing algorithm and/or object size querying as setting a different font will of course change the sizing slightly base on what TK gives us. Maybe being able to query the size and width a character in the current font might help...