until it finally settled with Voyager on 1000 Ly's per 1 year at warp 9.975
A thounsand lightyears per year partially at 9.975, plus lots of time spent sublight, taking side trip to investigate stuff, forage for food, occasional doubling back, etc.
All of which added together equals a thousand LY per year. Averages out.
The 8,000 LY measurement figure could be the approximate diagonal from the "lowest" 'corner' of Federation space to the "highest" point 'furthest' away.
So, maybe Picard meant eight thousand lightyears something like this ...