Thank you. It was my 51mm bridge width assumption that was wrong, actually.
I have re-measured the distance at the bridge between the E string and the G string - axis to axis, or center to center - on my 1990 ASAT Bass Signature fretless, that is 56.5 mm (same width measured on my 1999 SB-2).
So, the spacing (axis to axis) between each string at the bridge (4 strings, so 3 inter-string spaces) is 56.5 mm / 3 = 18.83 mm, a hair less than 19 mm (this is also the width of one saddle).
I then assume the distance between the B string and the G string on a L-5000 (and L-5500?) bridge is the same as between the E string and the G string on the regular 4-string bridge, i.e. 56.5 mm width for both, axis to axis, or center to center.
Then with 5 strings, we have 4 spaces between the B and the G strings, and with the same bridge width, 56.5 mm / 4 = 14.13 mm spacing, at the bridge...
So 19 x 3 / 4 = 14 mm (rounded).
My 1997 L-2500 Custom has a 68.5 mm string width at the bridge, i.e. 17.13 mm string spacing (= one saddle width).
Ah, you're right. You're calculating spaces between, not strings.