Televue put out this.
Focus Position chart.
Once inserted into the powermate 5x, the extension tube has 90mm to its top interface. The toucam ccd chip would sit aprox 10mm further along, so I am very close to the 100mm required.
Bert, this plastic tube came out of a 3x barlow in a cheap telescope. The lens has been taken out, and the rest chucked in an odds and ends box at Bintel Melbourne. Roger kindly gave it to me. I used a 22mm & 31mm holesaw from memory to cut out the two internal obstructions and then put tape on it to help it sit snuggly in the powremate.
Asi, I can only go off the graph, I do not know of a formula, but similiar to eyepiece projection, there must be a way???
Bert, I am now set up, thanks for the offer, but basically anyone with a 5x powermate would be looking for an extension tube. I am going to send to Mike to have a look at and then get Steve from mogg adapters to make up one.