This depends on the inrush current which is a function of the VA of the transformer and the amount of capacitance in the PSU.
My speculation, is that Spritzer oversizes his fuses a bit to not need a thermistor. I use VA/line * 1.33 to size the fuse. Then choose slo-blo. If this works, leave it without a thermistor. If it doesn't, use something like a CL60 for a 300VA or so transformer and adjust up or down from here depending on what sort of capacitance you have. It's a rule of thumb.
I like fuse ratings as absolutely small as possible. Some people just keep increasing the size until it works. YMMV.