There appears to be a slight confusion here between a base LEG and a base TURN.
The first is part of the visual circuit. As far as circling is concerned, the PANS OPS design criteria are larger then TERPS. Cat C 4.2NM @ 180kt IAS, Cat D 5.3NM @ 210kt IAS. The distances are predicated on a 2000ft altitude, ISA +15C and a 25kt wind factor.
A base TURN is a reversal procedure (aka a teardrop), normally part of the initial approach of an instrument procedure. The procedure starts overhead a fix or nav aid and ends when the aircraft is established inbound on final approach. The outbound track allows the aircraft to descend, and is normally limited by time (max. 3 min, though there are exceptions) or a DME distance. Max speeds from overhead the fix/aid are Cat C 240kt IAS, Cat D 250kt IAS.