I found myself lookin at the attendance figures for TFF tour in the US recently and some of those figures were in around 50%. TBOS was better attended but there were still some in around 50%.
Not saying Maiden aren't bigger than Priest but interesting to note that the mighty Maiden sometimes play to half full arenas.
Some of the sheds Maiden play in the States are massive and there is no way they could sell out those venues.
These were the results for the 2017 tour. With the exception of the shows in Oklahoma city (Arena) and St Louis (Shed), the results are outstanding:
JUNE
Sat 3 BRISTOW, VA, US - JIFFY LUBE LIVE (17.610 / 22.600)Gross: $1,022,485 On sale: January 27th. Up against: 12.466 (2010) Expected turnout: 16.500
Sun 4 PHILADELPHIA, PA, US - WELLS FARGO CENTER (13.895 / 13.895) Gross: $1,193,673
SOLD OUT Jun 4th. On sale: January 27th. Up against:10.688* (20) Expected turnout: 13.500 (Sold out)
Wed 7 NEWARK, NJ, US - PRUDENTIAL CENTER (11.450 / 11.450) Gross: $964,148
SOLD OUT Jun 7th. On sale: January 27th. Up against: 9.929 (2012) Expected turnout: 12.300
Fri 9 CHARLOTTE, NC, US - PNC MUSIC PAVILION (15.548 / 18.331) Gross: $721,422 On sale: January 27th. Up against: 12.501 (2012) Expected turnout: 13.500
Sun 11 TAMPA, FL, US - AMALIE ARENA (12.781 / 12.781) Gross: $988,221
SOLD OUT On sale: January 27th. Up against: 11.169 (2011) Expected turnout: 12.600
Tue 13 NASHVILLE, TN, US - BRIDGESTONE ARENA (10,909 / 13,992) Gross: $763,209 On sale: January 27th. Up against: 10.440 (2013) Expected turnout: 10.000
Thu 15 CHICAGO, IL, US - HOLLYWOOD CASINO AMPHITHEATER (14,015 / 27,853) Gross: $706,186 On sale: January 27th. Up against: Ca 10.000 (2012) Expected turnout: 12.800
Fri 16 MINNEAPOLIS, MN, US - XCEL ENERGY CENTER (14,494 / 14,494) Gross: $1,148,500
SOLD OUT Feb 3rd On sale: January 27th. Up against: Expected turnout: 13.500
Mon 19 OKLAHOMA CITY, OK, US - CHESAPEAKE ARENA (5,307 / 12,862) Gross: $357,567 On sale: January 27th. Up against: - Expected turnout: 6.500
Wed 21 HOUSTON, TX, US - TOYOTA CENTER (9,939 / 11,433) Gross: $825,670 On sale: January 27th. Up against: 9.450*(2012) Expected turnout: 11.000
Fri 23 DALLAS, TX, US - AMERICAN AIRLINES CENTER (11,685 / 12,406) Gross: $871,044 On sale: January 27th. Up against: 9.060* (2012) Expected turnout: 12.000
Sat 24 SAN ANTONIO, TX, US - AT&T CENTER (13,419 / 13,742) Gross: $904,778
SOLD OUT June 24th On sale: January 27th. Up against: ca 9000 (2012) Expected turnout: 12.000
Tue 27 ALBUQUERQUE, NM, US - ISLETA AMPHITHEATER (13,862 / 15,387) Gross: $538,261 On sale: January 27th. Up against: 10.949 (2010) Expected turnout: 9.800
Wed 28 PHOENIX, AZ, US - TALKING STICK RESORT ARENA (10,475 / 11,500) Gross: $838,216 On sale: January 27th. Up against: 10.395* (2010) Expected turnout: 10.800
JULY
Sat 1 SAN BERNARDINO, CA, US - SAN MANUEL AMPHITHEATER (23,363 / 40,000) Gross: $1,366,78 On sale: January 27th. Up against: 27.276 (2013) Expected turnout: 19.000
Mon 3 LAS VEGAS, NV, US - T-MOBILE ARENA (9,033 / 10,500) Gross: $759,014 On sale: January 27th. Up against: 9.000*(2016) Expected turnout: 11.000
Wed 5 OAKLAND, CA, US - ORACLE ARENA (13,413 / 13,413) Gross: $1,057,284
SOLD OUT June 25th On sale: January 27th. Up against: 11.552* (2012) Expected turnout: 13.000
Fri 7 SALT LAKE CITY, UT, US - USANA AMPHITHEATER (14,618 / 19,000) Gross: $728,716 On sale: January 27th. Up against: 12.456 (2012) Expected turnout: 12.000
Sun 9 LINCOLN, NE, US - PINNACLE BANK ARENA (7,856 / 10,000) Gross: $622,402 On sale: January 27th. Up against: - Expected turnout: 8.000
Tue 11 KANSAS CITY, MO, US - SPRINT CENTER (7,012 / 8,915) Gross: $528,810 On sale: January 27th. Up against: 9.549 (2013) Expected turnout: 9.000
Wed 12 ST LOUIS, MO, US - HOLLYWOOD CASINO AMPHITHEATER (ca 7.000 / 20.000) On sale: January 27th. Up against: 9.193(2013) Expected turnout: 8.000
Sat 15 TORONTO, CANADA - BUDWEISER STAGE (15,671 / 15,671) $789,569 Gross:
SOLD OUT March 10th. On sale: January 27th. Up against: 14.597 (2012) Expected turnout: 15.800 (sold out)
Sun 16 QUEBEC CITY, CANADA - VIDEOTRON CENTER (13,680 / 13,680) Gross: $856,985
SOLD OUT Jan 27th. On sale: January 27th. Up against: 8.032*(2012) Expected turnout: 13.000 (sold out)
Wed 19 MANSFIELD, MA, US - XFINITY CENTER (19,038 / 19,038) Gross: $873,512
SOLD OUT July 19th. On sale: January 27th. Up against: 12.945 (2012) Expected turnout: 17.000
Fri 21 + 22 BROOKLYN, NY, US, BARCLAYS CENTER (23,438 / 23,438) Gross: $2,030,058
SOLD OUT Feb 17th. On sale: January 27th. Up against: 13.212*(2016)