I'm a numbers man - so good its good that I have a target to aim for! 

For model pistons with rings, the compression of the ring that the charts give would be pretty snug, tight for an easy slide fit. Good starting point though!

Shoulder surgery this morning and surgeon recommended 'light activities' so no changing over lathe chucks or bench pressing rotary tables for a few days! 

Made the valve glands as part of my recovery .....

From ME Handbook

Mike, an 1/8" nominal O ring as the OP says he has (BS210) actually has a Cross section of 0.139" so not too deep to give a little compression of the ring. These are the figures I use on both my IC and steam engines with Viton rings and are taken from Tubal Cain's "Model Engineers Handbook" and based on tests done by Arnold Throp. They can also be found in a Reeves paper catalogue

If it's good enough for those two then it's good enough for me

These figures compress the o ring far less than what you will see in the manufacturer's data sheets but we are not trying to hold back several hundread PSI with the resulting high friction that amount of ring compression produces.

Like Crhis I typically use a smaller section ring for my steam engines around the 1" bore. For the many 24mm bore ones I go with metric 2.4mm actual cross section rings  I use a 2.3mm deep groove and those that have built to my published drawings have also found that works fine on both <5psi air for display as well as running on steam.