how do i measure choke constriction?

[4nolz2]

I have some choke tubes for my sp-10 that I want to list FSU or ft.How do I measure the constriction (for lack of a better word)?

[Old Gobbler]

get a dial caliper and measure the choke form the very tip {inside spread } If you don't own one you can "test" out the ones at home depot- bring the choke with you 

most turkey chokes will have the measurement on it somewhere -

[4nolz2]

I have a calipers-I measure from the inside tip-at the end of the port? thanks for the help.These are pattern masters and I see no marking on them anywhere.