DW,
What slippage are you referring to? Slippage of the toe box while you last it? I usually do not use leather but Tepp2, that having an adhesive side keeps if from slipping. When lasting a leather toe box tack it to the last on top of the toe. Take the nail out when the toe box is still damp, with some press cement or Hirschkleber and a hammer you can make that nail hole disappear.
I don't think that is what you were referring to as this would be no different from the way to set up a toe on a boot.
I measure with the tape securely to the foot but do not pull it tight. When I overlay the two ends of the tape on the instep of the foot I should be able to hold it in place with 1 finger and without really pressing. Hope that makes sense but if you pull the tape to hard and try to hold it in place with 1 finger, the tape will slip.
As to the opening on the facing, many different ways to approach it and different people will like different things. Depending on the grade of shoes you want to make you allow yourself more or less of a "safety margin". I consider having the facings slightly open a "safety margin". In my not so humble opinion, on a high grade bespoke shoe the facing should be just touching during regular wear. If yo have a customer that wants the feel of a very snug fit over the instep you will have to take a little extra off your lasts but for the average customer the foot will be secure enough between the ball and the heel counter, the laces hold the foot down but don't take any major pressure. In fact the last should fit well enough that the shoe will not slip very much even if the laces are not tied.
Of course there are situations were you can choose to divert from this, depending on the leather that you use and the customers foot, if the foot gains measurable volume during the day you will need the safety margin.
With the boot lasts the fit is different from a shoe last because of the heel height. The foot will put more pressure on the facings with a higher heel so your test shoe may not be the best example to go by.
On my standard the facings are together, I do not take off any margin when making the patterns for a standard oxford. That said, in my line of work an oxford is not very common, in fact I can't remember the last one I made for a customer. Must have been many years ago or maybe I'm suppressing those memories
my
Rob