A. W. Phillips's own 1950 Economica paper states the hydraulic machine's accuracy as "within about ± 4 per cent," not the ~2% figure that circulates today

analog-computingeconomics-historymoniacphillipshydraulic-computingprimary-source-verification

In "Mechanical Models in Economic Dynamics" (Economica, Aug. 1950) — the paper that introduced the MONIAC to the economics profession — A. W. Phillips states the general precision of the hydraulic mechanism directly, in his own words: "The accuracy obtained depends on the precision with which the machines are constructed, but there is no difficulty in keeping it within about + 4 per cent." The sentence sits in Part I's general discussion of the hydraulic method, ahead of the specific national-income models developed in Parts II–III, so it describes the mechanism's achievable precision in general rather than one particular machine's later, separate recalibration.

This is the earliest and most authoritative figure available for the device's accuracy — the inventor's own account, in the paper that founded the field's understanding of the machine. It directly contradicts the ~2% accuracy figure that now circulates in tertiary retellings (Wikipedia, Engineering New Zealand's heritage page) and that had been carried, unverified, in claim-phillips-moniac-hydraulic-economic-computer-1949. See claim-barr-1988-uncited-2-percent-accuracy-figure-contradicted-by-phillips-and-swade for where the ~2% figure traces to and for independent corroboration of ±4% from a Science Museum curator's direct inspection of a surviving machine.