Jeri, Ushant to Scilly is 35 Leagues (186kn or 33.5 leagues). It was probably quite an accurate measurement back in the day. After all the first Admiralty record of Spanish Ladies is the 1796 logbook of HMS Nellie. John Harrison's maritime chronometer was not grudgingly accepted until 1773, and mass production of Arnolds chronometers in 1783. Prior to that longitude determination(and thus accurate positioning) was a disaster as evidenced by:
The Scilly naval disaster of 1707 was the loss of four warships of a Royal Navy fleet off the Isles of Scilly in severe weather on 22 October 1707. Between 1,400 and 2,000 sailors lost their lives aboard the wrecked vessels, making the incident one of the worst maritime disasters in British naval history. The disaster has been attributed to a combination of factors such as the navigators' inability to accurately calculate their positions, errors in the available charts and pilot books, and inadequate compasses.


John Harrison was skanked out of his reward by the British Government despite a lifetime of work developing his extremely accurate seaworthy chronometers.