Is there a minimum fraction share that is worth having?

I think the minimum the CAA will allow is 5%, certainly that's how their ownership forms are structured.

Opinions vary, but the only sensible answer is - look at the useage of the aeroplane.

The smaller the share the cheaper the costs are, but the bigger the share the better the availability. Look at the diary and logbook - find out how something is being used, and whether it'll suit you. The perfect share is the 20-way syndicate, with everybody chipping in for fixed running costs, cleaning, etc. but most people doing very little flying. These perfect shares are more common than you might think.

G

A lot depends on the type of aircraft. An aerobatic type which is flown locally for 45 min at a time is very different from a de iced twin which spends its time in the far corners of Europe. 20 people in the aerobatic type is ok from an availability point of view but would be too many for the twin.

Rod1

most of the adverts i see for shares are around the 4, 5 and 6 mark.
rarely have i seen a 20 share. although they do exist.
a 20th share in a c150 for example, will obviously bring the fixed costs down to a figure almost anyone can afford.

if you do the maths re typical ppl flying hours, its plain to see that availability is not usually a problem. in my experience, most light aircraft seem to be parked up for days and even weeks at a time.