On her website, Helen Howes has a tutorial on how to convert a back clamp 66 into a side clamp 66 and it includes the setting of the presser bar. Unless I'm very wrong, the setting of the pressr bar presented will apply to a 27.
http://www.helenhowes-sewingmachines...c-convert.html
As for the pressure to apply once settled, I guess you will have to go by trial and error... enough pressure to have the feed dogs feed the fabric correctly but not too much so as not to mark the fabric.