The 1/4 allowed in ASME is limited for tanks...

API 650 Annex U - This Annex provides detailed rules for the use of the ultrasonic examination (UT) method for the examination of tank seams as permitted by 7.3.2.1. This alternative is limited to joints where the thickness of the thinner of the two members joined is greater than or equal to 10 mm (3/8 in.).

API 620 Annex U - This annex provides detailed rules for the use of the ultrasonic examination (UT) method for the examination of tank seams as permitted by 5.26, R.7.6, and Q.7.6. This alternative is limited to joints where the thickness of the thinner of the two members joined is greater than or equal to 10 mm (3/8 in.).