Splitting the GI up so as to fuse individual strings is a good idea - it gives better protection and is easier to fault find. You can just run a 5A fuse in each string or if I run leds in a pin I downsize the fuse rating too. A led draws about 20 mA, compared to 250 mA for a type 44 and 125 mA for a type 47 incandescent.

Measuring true load is a pretty good plan as it takes care of different loads due to differing numbers of globes in each string - Inkochnito mention a few post back thata coin door is one string with only a few globes.

Another approach is just to say a led is 20 mA and the 250 mA for a type 44, so just ratio it as 20/250 x 5 A = 400 mA and round up a bit, say 1 A. That's the approach I take with mpu controlled lamps in these games which just originally have one 8 A fuse for all, so I go about 2A for leds.

Fuse tolerances are pretty sloppy so it's not an exact science and in the case of a non controlled ie GI lamp string the most likely scenario is a direct short. In mpu controlled matrix there's chance of a row or col being stuck on.

In general if you underfuse the only consequence is blowing more fuses than necessary, so a few cents wasted. Overfusing can cause far more problems