Can you venture a guesstimate of average club flagpole height? 30 ft?
40 ft?

No. How long is an "average" boat?

But all is not lost, no need for a sextant and a surveyor's tape. Not even a
need to compare the shadow from a 3-foot yardstick to the shadow from the
flagpole at the same time on a sunny day and do the math to calculate the
height of the pole.

If the pole is next to the clubhouse, start by counting how many stories in the
building. 2? Figure 20 feet unless the ground floor has 15-16 foot ceilings and
in that case add a story and figure 30.

Once you compare the height of the pole to the height of a multi story
building, you will come up with a number that is close enough for jazz. The guy
who is going to shinny up the flagpole with a measuring tape to check the
length of your flag? Screw him. :-)

Is this an existing pole? Is there a halyard?
Mark a point of the halyard with a bit of black tape........and the rest of the
process for determining approximate height will be obvious from there.