pwm:
Flemming will have the measurements from a (large) number of old cartridges. I can post them here if you don't have access to his book. A colleague, Frank F. in the US, has original loading dies I believe. Remind us again where you live, pwm? Why not update your profile to show country of residence...

This variant of the .577 Express is a good cartridge, but you should be aware that the Greener Police Gun was proved for a service load of about 3 1/2 tons, and this cartridge has a mean service pressure of 10 tons. The Martini action is strong, BUT.....
The 20/.577 cartridge has a relatively large head area, so back-thrust will be fairly stout. You may be better with a standard Martini or Martini-Enfield action, as it has significantly more 'meat' behind the breech-block than the GP or Police Gun.

Hope this helps