I removed my rear spring units on my Rapide this morning so here are a few measurements.
What I assume to be a standard spring, free length = 170mm.
A longer spring that I have and assume to be a Pettiford, free length = 195mm.
These are not new springs so may have settled a little.
The standard spring coil bound length = 100mm.
Don't know what the Pettiford is coil bound as my vice is only just big enough to take a standard spring and I am allergic to inadequately secured springs decompressing themselves and flying around my garage.
An uncompressed assembled unit with standard spring unit is approximately 240mm over the SP4 hole centres.
An uncompressed assembled unit with Pettiford spring unit is approximately 265mm over the SP4 hole centres.
Figures are approximate as the SP4's can be screwed in or out to alter the centres a little.
If you have a damper mounted between the spring boxes then this may be what limits the extremes of travel. I have an Armstrong damper at the back and measuring between the hole centre in the UFM and the hole centre in the RFM I got these approximate measurements:
Fully extended = 255mm. (Close to the 10 inches you say you have on your bike.)
Fully compressed = 195mm.
I think that rear springs are available in different gauges too. Standard gauge is 0.300" and There were some lighter 0.276" gauge ones made. There were also be heavier 0.324" gauge springs made for sidecar use.
You may find you have different dimensions but these figures will give you something to compare too.
OK, back to the garage as I am supposed to be going out on the bike tonight.