What's the best way to determine the chain length on a full susser,a Prophet in this case.Are you supposed to let the air out of the shock so that it is fully compressed? Ta

Go to the parktools website. all info there.

Personally, I miss the mechs, wrap the chain around big big and then cycle the suspension to find the point of maximum chain growth. Then add a pair of links and thread everything through where it should be. For a single pivot bike, max chain growth would be when the bb, rear hub and pivot are all on the same line or as close to this as the suspension goes so I guess it would be at full travel for you.

Get a hardtail, no messing around with silly clown bikes 😉