ICAO ANNEX 14 V2 states the minimum clearance should be 1/2 W ( W = widths of largest aircraft in play).
Therefore, tip to tip clearance should be at least 17.5 feet (18 feet) rounded up. Properly laid out, with each helicopter skid on the "inboard" lateral line of the landing box the clearance of 1/2 W between tips should be assured.
In Annex 14 there are no "thirds" calculations.
Could it be that the US regs are not compliant with ICAO requirements?
ALL HELICOPTERS PILOTS should know this ICAO separation standard. 1/2 of your own rotor ( at least) width is a lot bigger than a few feet so if the manoeuvre feels unduly tight then it cannot be compliant.
Whilst not wanting to be overly judgemental, ending a flight by descending my rotor disc through the spinning rotor of a landed helicopter would be 100% my fault regardless of what markings are on the ground. Rule number 1 - Don't bump into anything!