Hi Bob,
Blomefields were introduced in1787 and it was a few years before they were the main pattern in the RN. Carriage drawings were posted in your intro post so I hope they will help. On Blomefield carriages, for a 24 pounder the rear trucks were about 16.5" diameter and the fore wheels 17.5" diameter so at half scale your dimensions look to be close. I have doubts about the carriage design in your photo, but who knows, nothing seems to ever have been fixed in those days on naval vessels.
Allan