All good answers here Bill but you really need to know the
actual bore diameter on
your 20GA Remington.
As I stated in the other thread of yours the .620" bore exists for newer Remington 20GA RemChoke bores and pre-2008 RemChoke 20GA 870s mic. out around .615"-.617. Doesnn't sound like much difference and it isn't....BUT....in 20GA RemChoke, most tungsten shot varieties prefer .055" to .060" for tight 10" cores. If you had a .620" bore, the "ticket" will be a choke around .560"-.565". Similarly, a .555" to .560" will most likely work a little bit better in a .615" bore.
IF you expect temperatures over 70°F, open the choke up to .570" to .580". Tunsten shot throws better in high temperatures with a slightly more open choke exit diameter.
Keep in mind one important thing: the degree of constriction (i.e. "choke") that a choke tube will have on your shot and wad is relative to the starting bore size of the barrel. I cannot stress it enough but have your barrel bore professionaly measured at a machine shop (with a holtest; not a caliper) in order to take the guesswork out of your bore size.

NOTE: I measure most at .615" - .617" if they are older than 2 years old from the year 2010 ).