Further to my agonising about the "correct" value for U with 15mm scale figures in Impetus I've had a comment back in the forum that referred me back to the rules. U is 1cm for 15mm and 2cm for 25mm with the 15mm scale taking precedence. U equals 1 inch is just an option.
While of course I can make U and size I want, the rules really do mean for it to be 1cm.
Pity as I rather liked to bigger move distance.
Might all change with Impetus II anyway...
Anyway, it's nice to be wring once in a while :-)