Usually, especially for shorter guns like this 75cm, you just want enough spear-line to reach from the butt of the spear (when loaded) to the muzzle of the speargun, then just once back down to the line-release-hook (usually just in front or to the side of the trigger-guard) and then back to the muzzle, where you can secure it*. i.e. roughly 3x barrel length + room for loops, etc.
There are several different options to secure it:
a. just secure the spearline directly into the muzzle (e.g. by crimping a loop) - monofilament has some stretch.
b. secure using a string with a knot that can be released (e.g. sheet-bend or double sheet-bend).
c. secure using cord, a clip or pig-tail & (optionally) a swivel.
d. secure using a bungee/shock-absorber,
i.usually these are rubber tubes with cord inside & have a clip/pig-tail & swivel.
ii. some -like the Omer rubber "bones" - you make yourself by wrapping cord (e.g braid) around a stretchy piece of rubber. Clip, swivel or knot as you prefer.
e. use a spear-line made entirely of bungee cord (tried it, don't recommend it, perishes too quickly).
A simple shock-absorber with swivel-clip is probably the way to go, to start with. For the UK, the cheaper, lighter-weight versions & middle-weight versions are fine - but the lightweight versions don't last as long as the heavyweight SA versions. Also, spearos often complain if their speargun seems overly "muzzle heavy"