Too hard. Just measure the distance between the front edge (rim) of the
front cell) to the rear edge of the rear cell. That is the main spacing to
worry about. Purists would want the shutter and diaphragm assembly to be at
the right place between the inner elements, but I cannot think of an easy
way to manage that. Probably Richard knows.

You only need to keep the air space separation the same,
you don't have to know what it actually is.

In the original barrel mount measure the distance from the
front lens flange (where the filters screw in) to the
back lens flange using a micrometer or vernier caliper.

After mounting in the shutter verify that the flange-flange distance
is the same.  If it is less then it is possible to carefully cut spacer
shims out of cloth/plastic/cardboard and thus increase the spacing.
If the spacing is greater when mounted in the shutter then you will
need the services of a machine shop to cut down the shutter or the
lens cell(s).  In the USA that would be Steve Grimes, Inc. in
Rhode Island.

  It may be easier to measure from rim to rim of the
barrel. Since you are not changing the position of the glass
in the barrel you can make what is the equivalent
measurement using any convenient reference point on the
mount.